A method and apparatus are disclosed from the perspective of a device to perform sidelink communication. In one embodiment, the method includes the device receiving a sidelink grant from a network node, wherein the sidelink grant schedules or assigns multiple sidelink resources. The method further includes the device generating a data packet comprising or multiplexing sidelink data from Sidelink (SL) logical channel(s) with SL Hybrid Automatic Request (HARD) feedback enabled. The method further includes the device performing two sidelink transmissions for the data packet on two adjacent, neighbor, or consecutive sidelink resources among the multiple sidelink resources if a time gap of the two adjacent, neighbor, or consecutive sidelink resources is larger than or equal to a minimum time gap.