A method and an apparatus are provided in this invention to select an optimal swapping technique in discrete multi-tone system. The algorithm for performing gain-swapping is also proposed in the present invention. A swapping technique is selected from gain-swapping and a combination of bit-swapping and gain-swapping based on two index values such that the difference between the maximum mean square error (MSEmax) and the minimum mean square error (MSEmin) is minimized and the gain factor constraints are met. The first index value I is representative of range of improvement when adopting the gain-swapping as the swapping technique, and the second index value J is representative of range of improvement when adopting a combination of the gain-swapping and the bit-swapping as the swapping technique.