An HARQ method for guaranteeing a QoS in a wireless communication system is provided. In a transmitting apparatus using a HARQ for transmitting packet data in a wireless communication system, a retransmission controller determines the size of redundancy information for correcting transmission errors in packet data, and a transmitter transmits the redundancy information of the determined size upon request for a retransmission of the packet data. Transmission of redundancy information of a variable size at each retransmission satisfies the QoS required for each user, maximizes a data throughput, and allows for the efficient reception of an ACK or a NACK for the redundancy information.