A method and apparatus are disclosed, from the perspective of the UE, for deriving UE transmit power. In one embodiment, the method includes the UE deriving a first pathloss value measured from a first reference signal. In addition, the method includes the UE deriving a second pathloss value measured from a second reference signal. The method also includes the UE transmitting a first UL transmission wherein the UL transmit power of the first UL transmission is derived from the first pathloss value. The method further includes the UE transmitting a second UL transmission wherein the UL transmit power of the second UL transmission is derived from the second pathloss value.