The present invention provides an apparatus and method for transmitting and receiving packet data in a mobile communication system supporting Hybrid Automatic Repeat reQuest (HARQ), and in particular, provides a method and apparatus for performing HARQ retransmission on a packet received through persistent resources where no HARQ process identifier is transmitted. The invention provides HARQ soft combining 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, the invention controls to determine with which packet an arbitrary retransmission packet should be combined, thereby allowing the corresponding processor to perform correct soft combining. Therefore, the invention can improve HARQ performance of persistent resources without increasing complexity of the reception apparatus.