The transmission power of a retransmission packet is controlled in the event of a packet retransmission request being sent from a receiving side in a wireless mobile communication system supporting hybrid automatic retransmission request (HARQ) transmission. In a method of controlling transmission power of retransmission packets by a mobile terminal in a wireless mobile communication system supporting hybrid automatic retransmission request (HARQ) transmission, the method includes steps of receiving, from a base station, gain information associated with the sequence of the retransmission packets; determining, using the gain information, transmission power of one of the retransmission packets; and transmitting the retransmission packet with the determined transmission power.