The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for performing RLC retransmission based on contention-based PUSCH in a wireless communication system, the method comprising: transmitting a MAC PDU including at least one RLC PDU received from a RLC entity and an identifier of the UE using a CB grant to an eNB; determining whether the MAC PDU transmission using the CB grant is failed or not; indicating result of the determining for the MAC PDU transmission using the CB grant; receiving the at least one RLC PDU at a next transmission opportunity if the MAC PDU transmission using the CB grant is failed, without considering that RLC status report including NACK for the at least one RLC PDU is not received.