Method and apparatus for constrained mode signaling are disclosed. According to one method, a plurality of mode syntaxes are used to signal a current mode used for the current block. If one or more of the plurality of candidate modes are unavailable according to one or more conditions related to the current block, signaling a mode syntax is skipped at the video encoder side or parsing the mode syntax is skipped at the video decoder side. In one embodiment, a regular flag indicating on or off of a regular Merge mode or Skip mode is not signaled if Merge mode with MVD (MMVD) mode, Affine mode, subblock Temporal Motion Vector Prediction (SbTMVP) mode, Combined Inter and Intra Prediction (CIIP) Merge mode, and Triangle Prediction Mode (TPM) are disabled as indicated by an SPS flag.