An apparatus and method for transmitting and receiving packet data in a mobile communication system supporting Hybrid Automatic Repeat reQuest (HARQ), e.g., a method and apparatus for performing HARQ retransmission on a packet received through persistent resources where no HARQ process identifier is transmitted. HARQ soft combining is performed by providing a mapping relation between a packet received through persistent resources and a retransmitted HARQ packet. In addition, when there are several HARQ packets received through persistent resources at an arbitrary timing, which packet an arbitrary retransmission packet should be combined is determined, thereby enabling the corresponding processor to perform correct soft combining. This improves HARQ performance of persistent resources without increasing complexity of the reception apparatus.