diff --git a/protocol/src/types/block.rs b/protocol/src/types/block.rs
index b4b4adc9a..6fd45ed11 100644
--- a/protocol/src/types/block.rs
+++ b/protocol/src/types/block.rs
@@ -219,6 +219,8 @@ pub struct Header {
     pub number:                   BlockNumber,
     pub gas_used:                 U256,
     pub gas_limit:                U256,
+    /// Extra data for the block header
+    /// The first index of extra_data is used to store hardfork information: `HardforkInfoInner`
     pub extra_data:               Vec<ExtraData>,
     pub base_fee_per_gas:         U256,
     pub proof:                    Proof,