Ab initio calculations on the reaction of the CH3 radical with H2CO were carried out at the CCSD(T)/ 6-311G(2df,p)//B3LYP/6-311+G(d,p) level. Three possible reaction pathways, the H-abstraction pathway RI, the C-addition pathway RII, and the O-addition pathway RIII, were considered. The entrance barrier heights of the three pathways at the CCSD(T) level are 40.20, 34.02, and 69.45 kJ mol(-1), respectively. Although the C-addition pathway has the lowest entrance barrier height, calculated rate constants using canonical variational transition state theory incorporating with small-curvature tunneling correction (CVT/SCT) show that the H-abstraction pathway is the fastest reaction channel in a wide temperature region of 300-2000 K. The C-addition pathway has considerable contribution only at the low-temperature region. The O-addition pathway is too slow and can be ruled out over the whole temperature region.