The reaction of CH3 radical with molecular O-2 has been investigated by ab initio molecular orbital theory and variational transition state theory calculations. The detailed potential energy surfaces, including the crossing seams between the PES, located by means of the intrinsic reaction coordinate approach are presented. The rate constants for the association and product formation channels have been calculated and compared with the experimental data. Under the atmospheric pressure condition, the association reaction (a) producing CH3O2 dominates reaction below 1500 K. The branching probabilities for channels (b) and (c) producing CH2O+OH and CH3O+O, respectively, have been calculated and compared; channel (b) is predicted to be dominant below 2000 K with the rate constant k(b)=1.14x10(-22)T(2.86)exp(-5120/T) cm(3) molecule(-1) s(-1). Over 2000 K, channel (c) becomes competitive; its rate constant could be represented by k(c)=1.01x10(-16)T(1.54) exp(-13 280/T) cm(3) molecule(-1) s(-1) in the temperature range of 1000-3000 K. In addition, the most exothermic products, CHO+H2O, were found to be kinetically inaccessible because of the large barrier, 47.4 kcal/mol above the reactants.