Provided are a method for adjusting a contention window size of a UE in a wireless communication system and the UE using the method. The method includes transmitting first data based on an AUL transmission to a BS, transmitting second data based on a grant-based uplink transmission to the BS, receiving first ACK/NACK information for the first data and second ACK/NACK information for the second data through an AUL-DFI, and adjusting the contention window size based on at least one of the first ACK/NACK information and the second ACK/NACK information included in the AUL-DFI. When the first data includes only one TB and the second data includes a plurality of TBs, the second ACK/NACK information is ACK if ACK/NACK for at least one of the plurality of TBs is ACK and the second ACK/NACK information is NACK if ACK/NACKs for all of the plurality of TBs are all NACKs.