The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for processing a PDCP reordering function in a dual connectivity system, the method comprising: generating a MAC PDU (Medium Access Control Protocol Data Unit) if both of a PHR (Power Headroom Reporting) and a sidelink BSR (Buffer Status Reporting) are generated while the UE communicates with other UEs directly using a sidelink; and transmitting the MAC PDU, wherein the PHR is prioritized over the sidelink BSR when the UE prioritizes between the PHR and the sidelink BSR in the generated MAC PDU.