In this study, the reaction mechanism and kinetics of the hydrogen abstraction and addition reactions of NCO with HCHO in the absence and presence of water have been investigated theoretically for the first time. Our theoretical results indicate that direct hydrogen abstraction is favored in the formation of HNCO instead of HOCN and the addition pathways are negligible with and without water. The rate constants calculated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/6-311++G(3df,3pd) level with zero-point energy correction range from 1.60 × 10(-12) to 4.99 × 10(-12) cm(3) molecule(-1) s(-1) between 220 and 769 K without water and are in good agreement with the available experimental values. However, with the inclusion of water, the rate constants are slower by 2-8 orders of magnitude than those of the reaction without water. Accordingly, the effect of water on the reaction of NCO with HCHO is negligible in the atmosphere.