The mechanism for the reaction of CH3 radical with O atom has been investigated theoretically at the B3LYP/6-311++ G(d,p)//CCSD(T)/ 6-311++G(d,p) level.The geometries of reactants,intermediates,transition states and products for CH3+O reaction ware calculated at the B3LYP/6-311++G(d,p) level.The transition states and intermediates of the reaction were verified by frequency analysis.The intrinstic reaction coordinate(IRC) calculations at the same level were also carried out to check the connection between all the critical structures located on the potential energy surface.Single point energies were calculated on the CCSD(T)/6-311++G(d,p) level.The results show that the reaction system of the title reaction involves many channels,producting H2CO+H,HCO+H2,H+CO+H2和 OH+CH2 products,respectively.The c5 is the dominant reaction channel.which agrees with the experimental.