The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for processing received PDCP PDUs for D2D communication system, the method comprising: establishing a PDCP (Packet Data Convergence Protocol) entity for transmitting a PDCP PDU (Protocol Data Unit) to a peer UE; receiving a PDCP SDU (Service Data Unit) from an upper layer; setting a COUNT for the PDCP SDU using a fixed value and a PDCP Sequence Number (SN) associated with the PDCP SDU; ciphering the PDCP SDU using the COUNT; generating a PDCP PDU including the PDCP SDU and a header including the PDCP SN of the PDCP SDU; and transmitting the PDCP PDU to the peer UE.