A method and apparatus for deprioritizing duplicated packet transmission in a wireless communication system is provided. A user equipment (UE) generates a first packet, a second packet and a third packet. The third packet is duplication of the second packet. The UE deprioritizes the third packet under the first packet when the second packet is prioritized over the first packet. The first packet may be a packet for uplink transmission, and the second packet and the third packet may be packets for sidelink transmission.