The present invention relates to a method of transmitting uplink (UL) data by a user equipment (UE) in a wireless communication system. In particular, the method includes the steps of: upon transmitting the UL data based on a configured grant (CG) configuration related to a Hybrid-ARQ (HARQ) process, starting a first timer related to the HARQ process and a second timer related to the HARQ process; and performing retransmission of the UL data based on the HARQ process, wherein an UL resource on which the retransmission is performed is determined based on whether a UL resource of the CG configuration is configured or not between an expiration time point of the first timer and an expiration time point of the second timer.