Main/Side (M/S) coding is a module deigned to reduce the channel irrelevancy and redundancies for multichannel coding in current audio standards. However, the success of this module relies on the careful design on four aspects: M/S decision, psychoacoustic model for the M/S channels, bits allocated between the M/S channels, and bit allocation in the M/S channels. This paper presents an efficient M/S method well designed from the four aspects. We conduct the experiments through the MP3 encoder to show the better quality and complexity than the Lame.