The present invention discloses a transmission method and apparatus for a wireless communication system. The data transmission method comprises the following steps. A first data mapped in a QAM constellation pattern is transmitted in a first transmission. And a second data, which is reversion of the first data and mapped in said QAM constellation pattern, is retransmitted. In embodiment, the interleaved data is partially swapped based on a predetermined swapping pattern. In another embodiment, first interleaved data and second interleaved data are multiplexed per N bits, N is an integer larger than 1. Therefore, improvement of reliability at first transmission or retransmission in a wireless communication system can be achieved.