diff --git a/src/core/mac/sub_mac.cpp b/src/core/mac/sub_mac.cpp
index 10f89ce90..d44f234c7 100644
--- a/src/core/mac/sub_mac.cpp
+++ b/src/core/mac/sub_mac.cpp
@@ -969,23 +969,6 @@ void SubMac::SetCslTimeout(uint32_t aTimeout)
     mCslTimeout = aTimeout;
 }
 
-void SubMac::FillCsl(Frame &aFrame)
-{
-    uint8_t *cur = aFrame.GetHeaderIe(Frame::kHeaderIeCsl);
-
-    if (cur != nullptr)
-    {
-        CslIe *csl = reinterpret_cast<CslIe *>(cur + sizeof(HeaderIe));
-
-        csl->SetPeriod(mCslPeriod);
-        csl->SetPhase(GetCslPhase());
-#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
-        otLogDebgMac("%10u:FillCsl() seq=%u phase=%hu", static_cast<uint32_t>(otPlatTimeGet()), aFrame.GetSequence(),
-                     csl->GetPhase());
-#endif
-    }
-}
-
 void SubMac::HandleCslTimer(Timer &aTimer)
 {
     aTimer.GetOwner<SubMac>().HandleCslTimer();
@@ -1031,23 +1014,6 @@ void SubMac::HandleCslTimer(void)
         break;
     }
 }
-
-uint16_t SubMac::GetCslPhase(void) const
-{
-    TimeMicro now = TimerMicro::GetNow();
-    uint32_t  delta;
-
-    if (mCslSampleTime < now)
-    {
-        delta = mCslSampleTime + mCslPeriod * kUsPerTenSymbols - now;
-    }
-    else
-    {
-        delta = mCslSampleTime - now;
-    }
-
-    return static_cast<uint16_t>(delta / kUsPerTenSymbols);
-}
 #endif // OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
 
 } // namespace Mac
diff --git a/src/core/mac/sub_mac.hpp b/src/core/mac/sub_mac.hpp
index 98a6c23a0..ff9646d9b 100644
--- a/src/core/mac/sub_mac.hpp
+++ b/src/core/mac/sub_mac.hpp
@@ -438,14 +438,6 @@ public:
      *
      */
     void SetCslTimeout(uint32_t aTimeout);
-
-    /**
-     * This method fills the CSL parameter to the frame.
-     *
-     * @param[inout]    aFrame  A reference to the frame to fill CSL parameter.
-     *
-     */
-    void FillCsl(Frame &aFrame);
 #endif // OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
 
     /**
@@ -504,7 +496,6 @@ private:
 #if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
     static void HandleCslTimer(Timer &aTimer);
     void        HandleCslTimer(void);
-    uint16_t    GetCslPhase(void) const;
 #endif
 
     enum