diff --git a/CHANGELOG.md b/CHANGELOG.md
index d48e5fd..2c9ce32 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 ### Features
 
 * Add PartialEq derived macro to Error in order to be able to test error cases
+* Allow `[no_alloc]` for faster-hex (without any feature), hex_string(_upper)() rely on heapless:String
 
 # [0.9.0](https://github.com/nervosnetwork/faster-hex/compare/v0.9.0..v0.8.2) (2023-11-22)
 Re create `v0.9.0`, since `v0.8.2` introduced a [break change](https://github.com/nervosnetwork/faster-hex/issues/43#issuecomment-1822551961), 
diff --git a/Cargo.toml b/Cargo.toml
index 625174b..39758f2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -17,15 +17,16 @@ exclude = [
 ]
 
 [dependencies]
-serde = { version = "1.0", optional = true }
+serde = { version = "1.0", default-features = false, optional = true }
 
 [features]
 default = ["std", "serde"]
-std = ["alloc"]
+std = ["alloc", "serde?/std"]
 alloc = []
-serde = ["dep:serde"]
-
+serde = ["dep:serde", "alloc"]
 
+[target.'cfg(not(feature = "alloc"))'.dependencies]
+heapless = { version = "0.8" }
 
 [dev-dependencies]
 criterion = "0.3"
diff --git a/src/decode.rs b/src/decode.rs
index 188d1f6..be8ea59 100644
--- a/src/decode.rs
+++ b/src/decode.rs
@@ -313,18 +313,25 @@ mod tests {
     };
     use proptest::proptest;
 
+    #[cfg(not(feature = "alloc"))]
+    const CAPACITY: usize = 128;
+
     fn _test_decode_fallback(s: &String) {
         let len = s.as_bytes().len();
         let mut dst = Vec::with_capacity(len);
         dst.resize(len, 0);
 
+        #[cfg(feature = "alloc")]
         let hex_string = hex_string(s.as_bytes());
+        #[cfg(not(feature = "alloc"))]
+        let hex_string = hex_string::<CAPACITY>(s.as_bytes());
 
         hex_decode_fallback(hex_string.as_bytes(), &mut dst);
 
         assert_eq!(&dst[..], s.as_bytes());
     }
 
+    #[cfg(feature = "alloc")]
     proptest! {
         #[test]
         fn test_decode_fallback(ref s in ".+") {
@@ -332,6 +339,14 @@ mod tests {
         }
     }
 
+    #[cfg(not(feature = "alloc"))]
+    proptest! {
+        #[test]
+        fn test_decode_fallback(ref s in ".{1,16}") {
+            _test_decode_fallback(s);
+        }
+    }
+
     fn _test_check_fallback_true(s: &String) {
         assert!(hex_check_fallback(s.as_bytes()));
         match (
diff --git a/src/encode.rs b/src/encode.rs
index ff56836..5095778 100644
--- a/src/encode.rs
+++ b/src/encode.rs
@@ -6,12 +6,15 @@ use core::arch::x86_64::*;
 #[cfg(feature = "alloc")]
 use alloc::{string::String, vec};
 
+#[cfg(not(feature = "alloc"))]
+use heapless::{String, Vec};
+
 use crate::error::Error;
 
 static TABLE_LOWER: &[u8] = b"0123456789abcdef";
 static TABLE_UPPER: &[u8] = b"0123456789ABCDEF";
 
-#[cfg(any(feature = "alloc", test))]
+#[cfg(feature = "alloc")]
 fn hex_string_custom_case(src: &[u8], upper_case: bool) -> String {
     let mut buffer = vec![0; src.len() * 2];
     if upper_case {
@@ -28,16 +31,46 @@ fn hex_string_custom_case(src: &[u8], upper_case: bool) -> String {
     }
 }
 
-#[cfg(any(feature = "alloc", test))]
+#[cfg(not(feature = "alloc"))]
+fn hex_string_custom_case<const N: usize>(src: &[u8], upper_case: bool) -> String<N> {
+    let mut buffer = Vec::<_, N>::new();
+    buffer
+        .resize(src.len() * 2, 0)
+        .expect("String<N> capacity too short");
+    if upper_case {
+        hex_encode_upper(src, &mut buffer).expect("hex_string");
+    } else {
+        hex_encode(src, &mut buffer).expect("hex_string");
+    }
+
+    if cfg!(debug_assertions) {
+        String::from_utf8(buffer).unwrap()
+    } else {
+        // Saftey: We just wrote valid utf8 hex string into the dst
+        unsafe { String::from_utf8_unchecked(buffer) }
+    }
+}
+
+#[cfg(feature = "alloc")]
 pub fn hex_string(src: &[u8]) -> String {
     hex_string_custom_case(src, false)
 }
 
-#[cfg(any(feature = "alloc", test))]
+#[cfg(not(feature = "alloc"))]
+pub fn hex_string<const N: usize>(src: &[u8]) -> String<N> {
+    hex_string_custom_case(src, false)
+}
+
+#[cfg(feature = "alloc")]
 pub fn hex_string_upper(src: &[u8]) -> String {
     hex_string_custom_case(src, true)
 }
 
+#[cfg(not(feature = "alloc"))]
+pub fn hex_string_upper<const N: usize>(src: &[u8]) -> String<N> {
+    hex_string_custom_case(src, true)
+}
+
 pub fn hex_encode_custom<'a>(
     src: &[u8],
     dst: &'a mut [u8],
diff --git a/src/lib.rs b/src/lib.rs
index b34ece3..d4acf1e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -15,10 +15,9 @@ pub use crate::decode::{
     hex_decode_unchecked,
 };
 pub use crate::encode::{
-    hex_encode, hex_encode_fallback, hex_encode_upper, hex_encode_upper_fallback,
+    hex_encode, hex_encode_fallback, hex_encode_upper, hex_encode_upper_fallback, hex_string,
+    hex_string_upper,
 };
-#[cfg(feature = "alloc")]
-pub use crate::encode::{hex_string, hex_string_upper};
 
 pub use crate::error::Error;
 
@@ -135,6 +134,9 @@ mod tests {
     use crate::{hex_encode_upper, hex_string_upper, vectorization_support, Vectorization};
     use proptest::proptest;
 
+    #[cfg(not(feature = "alloc"))]
+    const CAPACITY: usize = 128;
+
     #[test]
     fn test_feature_detection() {
         let vector_support = vectorization_support();
@@ -158,22 +160,29 @@ mod tests {
         {
             let encode = &*hex_encode(s.as_bytes(), &mut buffer).unwrap();
 
+            #[cfg(feature = "alloc")]
             let hex_string = hex_string(s.as_bytes());
+            #[cfg(not(feature = "alloc"))]
+            let hex_string = hex_string::<CAPACITY>(s.as_bytes());
 
             assert_eq!(encode, hex::encode(s));
-            assert_eq!(hex_string, hex::encode(s));
+            assert_eq!(hex_string.as_str(), hex::encode(s));
         }
 
         {
             let encode_upper = &*hex_encode_upper(s.as_bytes(), &mut buffer).unwrap();
 
+            #[cfg(feature = "alloc")]
             let hex_string_upper = hex_string_upper(s.as_bytes());
+            #[cfg(not(feature = "alloc"))]
+            let hex_string_upper = hex_string_upper::<CAPACITY>(s.as_bytes());
 
             assert_eq!(encode_upper, hex::encode_upper(s));
-            assert_eq!(hex_string_upper, hex::encode_upper(s));
+            assert_eq!(hex_string_upper.as_str(), hex::encode_upper(s));
         }
     }
 
+    #[cfg(feature = "alloc")]
     proptest! {
         #[test]
         fn test_hex_encode(ref s in ".*") {
@@ -181,13 +190,23 @@ mod tests {
         }
     }
 
+    #[cfg(not(feature = "alloc"))]
+    proptest! {
+        #[test]
+        fn test_hex_encode(ref s in ".{0,16}") {
+            _test_hex_encode(s);
+        }
+    }
+
     fn _test_hex_decode(s: &String) {
         let len = s.as_bytes().len();
-
         {
             let mut dst = Vec::with_capacity(len);
             dst.resize(len, 0);
+            #[cfg(feature = "alloc")]
             let hex_string = hex_string(s.as_bytes());
+            #[cfg(not(feature = "alloc"))]
+            let hex_string = hex_string::<CAPACITY>(s.as_bytes());
 
             hex_decode(hex_string.as_bytes(), &mut dst).unwrap();
 
@@ -198,7 +217,10 @@ mod tests {
         {
             let mut dst = Vec::with_capacity(len);
             dst.resize(len, 0);
+            #[cfg(feature = "alloc")]
             let hex_string_upper = hex_string_upper(s.as_bytes());
+            #[cfg(not(feature = "alloc"))]
+            let hex_string_upper = hex_string_upper::<CAPACITY>(s.as_bytes());
 
             hex_decode_with_case(hex_string_upper.as_bytes(), &mut dst, CheckCase::Upper).unwrap();
 
@@ -206,6 +228,7 @@ mod tests {
         }
     }
 
+    #[cfg(feature = "alloc")]
     proptest! {
         #[test]
         fn test_hex_decode(ref s in ".+") {
@@ -213,6 +236,14 @@ mod tests {
         }
     }
 
+    #[cfg(not(feature = "alloc"))]
+    proptest! {
+        #[test]
+        fn test_hex_decode(ref s in ".{1,16}") {
+            _test_hex_decode(s);
+        }
+    }
+
     fn _test_hex_decode_check(s: &String, ok: bool) {
         let len = s.as_bytes().len();
         let mut dst = Vec::with_capacity(len / 2);
diff --git a/src/serde.rs b/src/serde.rs
index eeea577..89e328c 100644
--- a/src/serde.rs
+++ b/src/serde.rs
@@ -1,15 +1,16 @@
 #![warn(missing_docs)]
 
-use std::iter::FromIterator;
+use core::iter::FromIterator;
 
 mod internal {
     use crate::{
         decode::{hex_decode_with_case, CheckCase},
         encode::hex_encode_custom,
     };
-    use alloc::borrow::Cow;
+    #[cfg(feature = "alloc")]
+    use alloc::{borrow::Cow, format, string::ToString, vec};
+    use core::iter::FromIterator;
     use serde::{de::Error, Deserializer, Serializer};
-    use std::iter::FromIterator;
 
     pub(crate) fn serialize<S, T>(
         data: T,
@@ -39,7 +40,7 @@ mod internal {
 
         hex_encode_custom(src, &mut dst[dst_start..], matches!(case, CheckCase::Upper))
             .map_err(serde::ser::Error::custom)?;
-        serializer.serialize_str(unsafe { ::std::str::from_utf8_unchecked(&dst) })
+        serializer.serialize_str(unsafe { ::core::str::from_utf8_unchecked(&dst) })
     }
 
     pub(crate) fn deserialize<'de, D, T>(
@@ -102,7 +103,7 @@ macro_rules! faster_hex_serde_macros {
         pub mod $mod_name {
             use crate::decode::CheckCase;
             use crate::serde::internal;
-            use std::iter::FromIterator;
+            use core::iter::FromIterator;
 
             /// Serializes `data` as hex string
             pub fn serialize<S, T>(data: T, serializer: S) -> Result<S::Ok, S::Error>