The present invention relates to a method of transmitting a data unit by a packet data convergence protocol (PDCP) entity of a user equipment (UE) in a wireless communication system. Especially, the PDCP entity is associated with at least two radio link control (RLC) entities and is configured with PDCP duplication. Further, the method includes the steps of receiving a PDCP service data unit (SDU) from an upper layer; constructing a PDCP protocol data unit (PDU) using the PDCP SDU; duplicating the PDCP PDU based on the PDCP duplication being activated; and transmitting the PDCP PDU and at least one duplicated PDCP PDU to the at least two RLC entity, wherein, based on the PDCP duplication being activated, constructing the PDCP PDU comprises compressing the PDCP SDU.