Abstract: Two different mechanisms have been put experimently forward for the photoreaction, in which HNO and CH_2O under both irradiation conditions at λ_(EXC)≥645 nm or λ_(EXC)=345 nm can produce cis and trans nitrosomethanol. In present paper, we located the S_0, S_1 and T_1 transition state and traced the IRC's of the reaction based on the ab initio method with STO-3G basis set. Using the GUGA CI method, we also calculated reactive and exciting energies of the reaction system. Our theoretical results support one of the two possible mechanisms suggested by experimentalists. We showed that whether the reactants are irradiated by light of λ≥645 nm or by light of λ=345 nm, in the first step of the reaction, the product formed will be trans nitrosomethanol.