The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for constructing a MAC PDU for D2D communication system, the method comprising: constructing, by a first UE, a MAC PDU including a source ID, a target ID, a plurality of logical channel IDs and a plurality of MAC SDUs, wherein each MAC SDU is transmitted from the first UE identified by the source ID to each second UE identified by the target ID via a logical channel identified by one of the plurality of logical channel IDs; transmitting the MAC PDU to at least one second UE via an interface, wherein the interface is configured between the first UE and the at least one second UE directly.