The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for handling an unknown MAC PDU, the method comprising: receiving a MAC (Medium Access Control) PDU (Protocol Data Unit) including a LCID (Logical Channel Identifier); discarding a header field containing the LCID and a corresponding MAC SDU (Service Data Unit) indicated by the header field in the received MAC PDU if the LCID is not configured to the UE; and processing remaining parts of the MAC PDU after the header field and the corresponding MAC SDU are discarded from the MAC PDU.