Abstract: The complex potential energy surface for the reaction of·OH radical with CH3CN, including 2 intermediate complexes, 9 transition states, was theoretically probed at the CBS-QB3 level. The geometries and relative energies for various stationary points were determined. Based on the calculated CBS-QB3 potential energy surface, the possible reaction mechanism of·OH+CH3CN was proposed. The calculated results demonstrated that the formation of P1 (·CH2CN+H2O) was the dominant reaction channel. The rate constant (k1, cm3·molecule-1·s-1) of P1 was calculated by TS theory. Over the temperature range 250-3000 K, we predicted that the expression of k1 was k1(250-3000 K)=2.06×10^-20T^3.045exp(-780.00/T). By comparing with the obtained experimental values it was shown that the values of k1 were in od agreement with the experimental results over the temperature range 250-320 K. The calculated results indicated the formation of P1 (CH3CN+·OH) only needed a barrier of 14.2 kJ·mol-1. While the products ·CH2CN+H2O retrograded to the reactants CH3CN+·OH, an energy barrier of 111.2 kJ·mol-1 was required. These results suggested that the backward direction for the pathway of formation product P1 would be difficult in the ground electronic state.