diff --git a/specs/casper_sharding_v2.1.md b/specs/casper_sharding_v2.1.md
index 0c3b70f40..9e5f8c241 100644
--- a/specs/casper_sharding_v2.1.md
+++ b/specs/casper_sharding_v2.1.md
@@ -443,5 +443,14 @@ Slashing conditions may include:
     Proof of custody no secret reveal
     RANDAO leak
 
+# Appendix
+## Appendix A - Hash function
+The general hash function `hash(x)` in this specification is defined as: 
+
+`hash(x) := BLAKE2b-512(x)[0:32]`, where `BLAKE2b-512` (`blake2b512`) algorithm is defined in [RFC 7693](https://tools.ietf.org/html/rfc7693) and input `x` is bytes type.
+
+* `BLAKE2b-512` is the *default* `BLAKE2b` algorithm with 64-byte digest size. To get a 32-byte result, the general hash function output is defined as the leftmost `32` bytes of `BLAKE2b-512` hash output.
+* The design rationale is keeping using the default algorithm and avoiding too much dependency on external hash function libraries.
+
 ## Copyright
 Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).