A method and apparatus are disclosed from the perspective of a device to perform sidelink transmission. The method includes the device being in RRC (Radio Resource Control)-connected mode in Uu link. The method also includes the device being configured to use at least DL (Downlink) pathloss for sidelink power control. The method further includes the device deriving a first DL pathloss value for determining an uplink transmit power of one specific kind of uplink transmission. In addition, the method includes the device determining or deriving a sidelink transmit power based on the first DL pathloss value. Furthermore, the method includes the device performing a sidelink transmission to other device(s) with the sidelink transmit power.