diff --git a/src/core/thread/network_diagnostic_tlvs.hpp b/src/core/thread/network_diagnostic_tlvs.hpp
index 1dc0bcc89..76102bb07 100644
--- a/src/core/thread/network_diagnostic_tlvs.hpp
+++ b/src/core/thread/network_diagnostic_tlvs.hpp
@@ -157,6 +157,7 @@ public:
  * This class implements Extended Address TLV generation and parsing.
  *
  */
+OT_TOOL_PACKED_BEGIN
 class ExtMacAddressTlv : public NetworkDiagnosticTlv
 {
 public:
@@ -197,7 +198,7 @@ public:
 
 private:
     Mac::ExtAddress mMacAddr;
-};
+} OT_TOOL_PACKED_END;
 
 /**
  * This class implements Source Address TLV generation and parsing.