An apparatus and method for retransmitting a codeword packet in a system using Hybrid Automatic Repeat reQuest (HARQ) are provided. The HARQ retransmission method includes defining multiple mother codes, and selecting one of code rates of the mother codes as an initial transmission code rate according to channel condition, determining a retransmission unit for each of codeword packets coded with code rates corresponding to the mother codes and retransmitting corresponding codeword packets in units of the determined retransmission units every time there is a retransmission request.