A method and apparatus for flushing a hybrid automatic repeat request (HARQ) buffer in a wireless communication system is provided. A user equipment (UE) receives information on multiple resource pools from a network, selects a first resource pool among the multiple resource pools, and stores a media access control (MAC) protocol data unit (PDU) in the HARQ buffer for transmission via the first resource pool. When the UE reselects a second resource pool among the multiple resource pools, the UE flushes the HARQ buffer.