Abstract：The reaction mechanism of CH3• radical with HNCO has been investigated by B3LYP method of density functional theory, while the geometries and harmonic frequencies of reactants, intermediates, transition states and products have been calculated at the B3LYP/6-311＋＋G(d,p) level. To obtain more accurate energy result, stationary point energies were calculated. The vibration analysis and the IRC analysis testified the authenticity of intermediates and transition states. Seven feasible reaction pathways of this reaction have been studied. The results indicate that the pathway CH3•＋HNCO→TS7→IM4→TS9→IM5 is the most energetically favorable and the major pathway. In the reaction system, the activation energy of the proton transfer process is not very high, and it can also happen.