The present disclosure relates to a method of receiving data units by a user equipment (UE) in a wireless communication system. In particular, the method includes the steps of: receiving the data unit during an active time of a first discontinuous reception (DRX) configuration related to a multicast transmission for a Hybrid Automatic Repeat and request (HARQ) process; transmitting a negative response to the network based on the data unit being not successfully decoded; starting a HARQ Round-trip Time (RTT) timer of a second DRX configuration related to a unicast transmission for the HARQ process; and monitoring a physical downlink control channel (PDCCH) related to a retransmitted data unit during an active time of the second DRX configuration, based on the HARQ RTT timer expiring