The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for determining result of transmission for contention-based PUSCH in a wireless communication system, the method comprising: transmitting a MAC PDU using a CB grant to an eNB; starting a timer when the UE transmits the MAC PDU using the CB grant; monitoring an ACK indication transmitted by the eNB while the timer is running; considering that the MAC PDU transmission using the CB grant is failed if the UE doesn't receive the ACK indication from the eNB until the timer expires; and retransmitting the MAC PDU to the eNB.