A data transmission method using an HARQ includes transmitting a first transmission block which is a portion of bit streams of a mother codeword, receiving a retransmission request with respect to the mother codeword, and transmitting a second transmission block which is a bit stream subsequent to the first transmission block, according to the retransmission request, wherein a modulation order obtained from a first range of a modulation order product code rate (MPR) is applied to the first transmission block, a modulation order obtained from a second range of the MPR according to the number of retransmissions is applied to the second transmission block, and the second range of the MPR is adjusted as much as a linear offset to the first range.