diff --git a/Volume-Server-API.md b/Volume-Server-API.md
index 17f356c..e44d3c2 100644
--- a/Volume-Server-API.md
+++ b/Volume-Server-API.md
@@ -42,7 +42,7 @@ The size returned is the size stored on SeaweedFS, sometimes the file is automat
 | ---- | -- | -- |
 | Content-Encoding: gzip|the uploaded content is already compressed to gzip | empty |
 | Content-Type | use the specified content type | "application/octet-stream" |
-| Content-MD5  | verify the uploaded content by MD5 | empty |
+| Content-MD5  | verify the uploaded content by MD5. MD5 digest needs to be encoded as base64 string. | empty |
 | Authorization | Json Web Token for writes issued by master | empty |
 | Seaweed-xxxxx | Any key-value pair with custom `Seaweed-` prefix. All the custom key-value data are stored as json, should be less than 64KB. The key will trim out the `Seaeweed-` prefix and then convert into a canonical header key. See https://golang.org/pkg/net/http/#CanonicalHeaderKey | empty |