The present invention relates to a method of performing uplink transmission by a user equipment (UE) in a wireless communication system. In particular, the method includes the steps of: receiving a configured grant (CG) activation command for a configured uplink grant; based on a size of the configured uplink grant not matching with a size of a pending protocol data unit (PDU) associated with an identified HARQ process, flushing a Hybrid Automatic Repeat Request (HARQ) buffer of the identified HARQ process; and transmitting a CG confirmation medium access control (MAC) control element (CE) using the identified HARQ process.