A method and apparatus for allocation of lower identifier (ID) and upper identifier (ID) for sidelink transmission in a wireless communication system is provided. A first wireless device (e.g., receiving user equipment (UE)) receives, via upper layer signaling from a second wireless device (e.g., transmitting UE), a lower ID associated with an upper ID and monitors sidelink control information (SCI) based on the lower ID and the upper ID. The first wireless device receives, from the second wireless device, a data unit scheduled by the SCI.