The present invention relates to a method of processing a data unit by a User Equipment (UE) in a wireless communication system. In particular, the method includes the steps of: configuring, by the UE, a radio bearer comprising a Packet Data Convergence Protocol (PDCP) entity, wherein the PDCP entity is related to two or more Radio Link Control (RLC) groups and at least one of the two or more RLC groups comprises two or more RLC entities; selecting, by the PDCP entity of the UE, one of the two or more RLC groups; and based on a RLC group comprising two or more RLC entities being selected from the two or more RLC groups, duplicating, by the PDCP entity of the UE, a PDCP Protocol Data Unit (PDU); and submitting, by the PDCP entity of the UE, the PDCP PDU to the two or more RLC entities belonging to the selected RLC group.