Methods and apparatuses for improving initialization of sidelink duplication reception in a wireless communication system are disclosed herein. In one method, a User Equipment (UE) receives a first Packet Data Convergence Protocol (PDCP) Protocol Data Unit (PDU) on a logical channel associated with a sidelink radio bearer (SLRB). The UE initializes a first state variable based on a partial Reordering_Window and a PDCP Sequence Number (SN) of the first received PDCP PDU, wherein the first state variable indicates the PDCP SN of a last PDCP Service Data Unit (SDU) delivered to the upper layers by a PDCP entity created for the SLRB, and the partial Reordering_Window is less than a Reordering_Window. The UE performs a re-ordering procedure for the first PDCP PDU using the initialized first state variable.