A series of key stationary points of potential energy surface (PES) for the CH_3S + O_2 system was calculated at the G3(MP2) level. The calculations reveal four intermediates, nine transition states and six product channels. With the stationary points and TST-RRKM theory, the reaction rates are calculated, gas phase reaction mechanism of CH_3S + O_2 is discussed. At low temperature, the reaction producing CH_3SOO is the main channel. At high temperature, the addition elimination mechanism giving products of CH_3 + SO_2 and H-abstraction mechanism giving products of CH_2S + HO_2 are dominant reaction channels, the other products might be neglected.

Key words: AB INITIO CALCULATION, methylthiyl, FREE RADICAL, REACTION MECHANISM