A method and apparatus are disclosed from the perspective of a second User Equipment (UE) for performing sidelink transmission to a first UE. In one embodiment, the method includes the second UE generating a MAC (Medium Access Control) CE (Control Element) with a fixed or (pre-defined) priority value, wherein the MAC CE is for transmitting to the first UE. The method further includes the second UE generating a MAC PDU (Protocol Data Unit) comprising the MAC CE and sidelink data from one or more sidelink logical channels, wherein the sidelink data is available for transmitting to the first UE. Furthermore, the method includes the second UE setting a priority field in a first SCI to a fixed or (pre-defined) priority value of the MAC CE. In addition, the method includes the second UE transmitting the first SCI to the first UE, wherein the first SCI schedules a first sidelink transmission for transmitting the MAC PDU.