The present invention relates to a method of transmitting a status report by a user equipment (UE) in a wireless communication system. Especially, the method includes the steps of receiving information about whether a Packet Data Convergence Protocol (PDCP) duplication for downlink is activated or not; receiving at least one data unit from a network; determining whether the status report related to the at least one data unit is triggered or not; based on whether the PDCP duplication for the downlink is activated or not; and transmitting the triggered status report related to the at least one data unit, wherein, based on the PDCP duplication for the downlink being activated, the status report related to the at least one data unit is not triggered.