The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for performing a buffer status reporting in a D2D communication system, the method comprising: configuring a plurality of sidelink logical channels, wherein each of the plurality of sidelink logical channels has an associated priority and belongs to a LCG, and one or more of LCGs belong to a ProSe Destination; generating a SL BSR MAC CE including buffer sizes of LCGs in decreasing order of a LCG priority, wherein the LCG priority of a LCG is determined as a highest priority of a logical channel among priorities of logical channels belonging to the LCG; and transmitting a MAC PDU including the SL BSR MAC CE.