The present disclosure relates to a method for performing an uplink (UL) transmission by user equipment in a wireless communication system. In particular, the method includes the steps of: receiving an UL grant associated with a first Hybrid Automatic Repeat Request (HARQ) process from a gNB; selecting a second HARQ process among the first HARQ process and the second HARQ process, when there is a first medium access control (MAC) protocol data unit (PDU) which is associated with the first HARQ process and is a pending MAC PDU; generating a second MAC PDU associated with the second HARQ process; and performing the uplink transmission of the second MAC PDU with an identifier (ID) of the second HARQ process.