diff --git a/src/core/api/joiner_api.cpp b/src/core/api/joiner_api.cpp
index 0b35e2604..4a1d8dffc 100644
--- a/src/core/api/joiner_api.cpp
+++ b/src/core/api/joiner_api.cpp
@@ -83,7 +83,7 @@ otError otJoinerSetDiscerner(otInstance *aInstance, otJoinerDiscerner *aDiscerne
     otError          error  = OT_ERROR_NONE;
     MeshCoP::Joiner &joiner = static_cast<Instance *>(aInstance)->Get<MeshCoP::Joiner>();
 
-    if (aDiscerner != NULL)
+    if (aDiscerner != nullptr)
     {
         error = joiner.SetDiscerner(*static_cast<const MeshCoP::JoinerDiscerner *>(aDiscerner));
     }
diff --git a/src/core/backbone_router/bbr_manager.hpp b/src/core/backbone_router/bbr_manager.hpp
index f55aa595f..bd68a110b 100644
--- a/src/core/backbone_router/bbr_manager.hpp
+++ b/src/core/backbone_router/bbr_manager.hpp
@@ -73,7 +73,7 @@ public:
      * Note: available only when `OPENTHREAD_CONFIG_REFERENCE_DEVICE_ENABLE` is enabled.
      *       Only used for test and certification.
      *
-     * @param[in] aMlIid    A pointer to the Mesh Local IID. If NULL, respond with @p aStatus for any
+     * @param[in] aMlIid    A pointer to the Mesh Local IID. If nullptr, respond with @p aStatus for any
      *                      coming DUA.req, otherwise only respond the one with matching @p aMlIid.
      * @param[in] aStatus   The status to respond.
      *
diff --git a/src/core/mac/link_raw.cpp b/src/core/mac/link_raw.cpp
index 9fa17ceed..dc0f5c22d 100644
--- a/src/core/mac/link_raw.cpp
+++ b/src/core/mac/link_raw.cpp
@@ -68,7 +68,7 @@ otError LinkRaw::SetReceiveDone(otLinkRawReceiveDone aCallback)
 {
     otError error = OT_ERROR_NONE;
 
-    otLogDebgMac("LinkRaw::Enabled(%s)", (aCallback != NULL ? "true" : "false"));
+    otLogDebgMac("LinkRaw::Enabled(%s)", (aCallback != nullptr ? "true" : "false"));
 
 #if OPENTHREAD_MTD || OPENTHREAD_FTD
     VerifyOrExit(!Get<ThreadNetif>().IsUp(), error = OT_ERROR_INVALID_STATE);
diff --git a/src/core/mac/link_raw.hpp b/src/core/mac/link_raw.hpp
index 5de185491..0f0c81975 100644
--- a/src/core/mac/link_raw.hpp
+++ b/src/core/mac/link_raw.hpp
@@ -71,13 +71,13 @@ public:
      * @returns true if enabled, false otherwise.
      *
      */
-    bool IsEnabled(void) const { return mReceiveDoneCallback != NULL; }
+    bool IsEnabled(void) const { return mReceiveDoneCallback != nullptr; }
 
     /**
      * This method enables/disables the raw link-layer.
      *
-     * @param[in]   aCallback    A pointer to a function called on receipt of a IEEE 802.15.4 frame, NULL to disable
-     * raw link-layer.
+     * @param[in]  aCallback  A pointer to a function called on receipt of a IEEE 802.15.4 frame, nullptr to disable
+     *                        raw link-layer.
      *
      *
      * @retval OT_ERROR_INVALID_STATE   Thread stack is enabled.
diff --git a/src/core/thread/dua_manager.cpp b/src/core/thread/dua_manager.cpp
index f8178af0f..bb12ff220 100644
--- a/src/core/thread/dua_manager.cpp
+++ b/src/core/thread/dua_manager.cpp
@@ -615,7 +615,7 @@ otError DuaManager::ProcessDuaResponse(Coap::Message &aMessage)
     {
         Child *child = Get<ChildTable>().GetChildAtIndex(mChildIndexDuaRegistering);
 
-        VerifyOrExit(child != NULL, error = OT_ERROR_NOT_FOUND);
+        VerifyOrExit(child != nullptr, error = OT_ERROR_NOT_FOUND);
         VerifyOrExit(child->HasIp6Address(target), error = OT_ERROR_NOT_FOUND);
 
         mRegisterCurrentChildIndex = false;
@@ -684,7 +684,7 @@ exit:
         otLogWarnDua("Sent ADDR_NTF for child %04x DUA %s Error %s", aChild.GetRloc16(),
                      aAddress.ToString().AsCString(), otThreadErrorToString(error));
 
-        if (message != NULL)
+        if (message != nullptr)
         {
             message->Free();
         }
diff --git a/src/ncp/ncp_base_radio.cpp b/src/ncp/ncp_base_radio.cpp
index 0d23a697f..8d3d4499d 100644
--- a/src/ncp/ncp_base_radio.cpp
+++ b/src/ncp/ncp_base_radio.cpp
@@ -348,7 +348,7 @@ template <> otError NcpBase::HandlePropertySet<SPINEL_PROP_PHY_ENABLED>(void)
 
     if (value == false)
     {
-        error = otLinkRawSetReceiveDone(mInstance, NULL);
+        error = otLinkRawSetReceiveDone(mInstance, nullptr);
     }
     else
     {
diff --git a/src/posix/platform/alarm.cpp b/src/posix/platform/alarm.cpp
index 0dfdddc0b..195a2719c 100644
--- a/src/posix/platform/alarm.cpp
+++ b/src/posix/platform/alarm.cpp
@@ -113,7 +113,7 @@ void platformAlarmInit(uint32_t aSpeedUpFactor, int aRealTimeSignal)
         sa.sa_sigaction = microTimerHandler;
         sigemptyset(&sa.sa_mask);
 
-        VerifyOrDie(sigaction(aRealTimeSignal, &sa, NULL) != -1, OT_EXIT_ERROR_ERRNO);
+        VerifyOrDie(sigaction(aRealTimeSignal, &sa, nullptr) != -1, OT_EXIT_ERROR_ERRNO);
 
         sev.sigev_notify          = SIGEV_SIGNAL;
         sev.sigev_signo           = aRealTimeSignal;
@@ -176,7 +176,7 @@ void otPlatAlarmMicroStartAt(otInstance *aInstance, uint32_t aT0, uint32_t aDt)
         its.it_interval.tv_sec  = 0;
         its.it_interval.tv_nsec = 0;
 
-        if (-1 == timer_settime(sMicroTimer, 0, &its, NULL))
+        if (-1 == timer_settime(sMicroTimer, 0, &its, nullptr))
         {
             otLogWarnPlat("Failed to update microsecond timer: %s", strerror(errno));
         }
@@ -195,7 +195,7 @@ void otPlatAlarmMicroStop(otInstance *aInstance)
     {
         struct itimerspec its = {{0, 0}, {0, 0}};
 
-        if (-1 == timer_settime(sMicroTimer, 0, &its, NULL))
+        if (-1 == timer_settime(sMicroTimer, 0, &its, nullptr))
         {
             otLogWarnPlat("Failed to stop microsecond timer: %s", strerror(errno));
         }