diff --git a/src/core/thread/link_metrics_tlvs.hpp b/src/core/thread/link_metrics_tlvs.hpp
index bec61cdde..56ff1cca6 100644
--- a/src/core/thread/link_metrics_tlvs.hpp
+++ b/src/core/thread/link_metrics_tlvs.hpp
@@ -153,7 +153,7 @@ public:
 
 private:
     uint8_t mMetricsTypeId;
-    union
+    union OT_TOOL_PACKED_FIELD
     {
         uint8_t  m8;
         uint32_t m32;