A method for retransmitting coded bits by a transmitter in response to a retransmission request from a receiver in a mobile communication system. The method comprises determining a modulation technique to be used at retransmission as a modulation technique having a lower modulation order than a modulation technique used at initial transmission; determining a puncturing pattern according to an HARQ (Hybrid Automatic Repeat Request) type, and selecting as many coded bits as the number of coded bits that can be symbol-mapped by the determined modulation technique, among the coded bits punctured by the determined puncturing pattern; and symbol-mapping the selected coded bits by the determined modulation technique, and transmitting the symbol-mapped coded bits to the receiver.