diff --git a/src/core/net/sntp_client.cpp b/src/core/net/sntp_client.cpp
index 4e397f3e4..c86b17157 100644
--- a/src/core/net/sntp_client.cpp
+++ b/src/core/net/sntp_client.cpp
@@ -48,6 +48,49 @@
 namespace ot {
 namespace Sntp {
 
+Header::Header(void)
+    : mFlags(kNtpVersion << kVersionOffset | kModeClient << kModeOffset)
+    , mStratum(0)
+    , mPoll(0)
+    , mPrecision(0)
+    , mRootDelay(0)
+    , mRootDispersion(0)
+    , mReferenceId(0)
+    , mReferenceTimestampSeconds(0)
+    , mReferenceTimestampFraction(0)
+    , mOriginateTimestampSeconds(0)
+    , mOriginateTimestampFraction(0)
+    , mReceiveTimestampSeconds(0)
+    , mReceiveTimestampFraction(0)
+    , mTransmitTimestampSeconds(0)
+    , mTransmitTimestampFraction(0)
+{
+}
+
+QueryMetadata::QueryMetadata(void)
+    : mTransmitTimestamp(0)
+    , mResponseHandler(NULL)
+    , mResponseContext(NULL)
+    , mTransmissionTime(0)
+    , mDestinationPort(0)
+    , mRetransmissionCount(0)
+{
+    mSourceAddress.Clear();
+    mDestinationAddress.Clear();
+}
+
+QueryMetadata::QueryMetadata(otSntpResponseHandler aHandler, void *aContext)
+    : mTransmitTimestamp(0)
+    , mResponseHandler(aHandler)
+    , mResponseContext(aContext)
+    , mTransmissionTime(0)
+    , mDestinationPort(0)
+    , mRetransmissionCount(0)
+{
+    mSourceAddress.Clear();
+    mDestinationAddress.Clear();
+}
+
 Client::Client(Ip6::Netif &aNetif)
     : mSocket(aNetif.Get<Ip6::Udp>())
     , mRetransmissionTimer(aNetif.GetInstance(), &Client::HandleRetransmissionTimer, this)
diff --git a/src/core/net/sntp_client.hpp b/src/core/net/sntp_client.hpp
index 879dc7a0e..9f56f3d68 100644
--- a/src/core/net/sntp_client.hpp
+++ b/src/core/net/sntp_client.hpp
@@ -60,13 +60,7 @@ public:
      * Default constructor for SNTP Header.
      *
      */
-    Header(void)
-    {
-        memset(this, 0, sizeof(*this));
-
-        // Set default value of flags field.
-        SetFlags(kNtpVersion << kVersionOffset | kModeClient << kModeOffset);
-    }
+    Header(void);
 
     /**
      * Defines supported SNTP modes.
@@ -430,7 +424,7 @@ public:
      * Default constructor for the object.
      *
      */
-    QueryMetadata(void) { memset(this, 0, sizeof(*this)); }
+    QueryMetadata(void);
 
     /**
      * This constructor initializes the object with specific values.
@@ -439,12 +433,7 @@ public:
      * @param[in]  aContext  Context for the handler function.
      *
      */
-    QueryMetadata(otSntpResponseHandler aHandler, void *aContext)
-    {
-        memset(this, 0, sizeof(*this));
-        mResponseHandler = aHandler;
-        mResponseContext = aContext;
-    }
+    QueryMetadata(otSntpResponseHandler aHandler, void *aContext);
 
     /**
      * This method appends request data to the message.