The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for avoiding padding only MAC PDU in a D2D communication system, the method comprising: receiving a sidelink grant from a base station (BS) when the TX UE communicates with a receiving UE (RX UE) directly; checking size of the received sidelink grant; and transmitting a Medium Access Control Protocol Data Unit (MAC PDU) including at least one byte of Radio Link Control Service Data Unit (RLC SDU) if the size of received sidelink grant is larger than a minimum size of MAC PDU including only MAC header and Radio Link Control (RLC) header for Sidelink Traffic Channel (STCH).