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: based on the UE transmitting the UL data on a UL resource of a configured grant (CG) configuration based on a Hybrid-ARQ (HARQ) process, starting a CG timer for the HARQ process; based on retransmission of the UL data based on the HARQ process not being performed until expiration of the CG timer, determining that a transmission of the UL data is not successful; and performing retransmission of the UL data based on the HARQ process after the expiration of the CG timer.