Graphical abstract Two figures are the Energy diagrams for the HO2+NH2 reaction paths on the singlet and triplet surface. As shown in figures, on the singlet potential surface, the most favorable association way starts with a barrierless addition of the HO2 to NH2 leading to HOO–NH2 (1im2 and 1im3). The product of OH and NH2O should be the major product, which obtained by a direct dissociation of the adduct 1im3 without an exit barrier. But on the triplet potential surface, DFT calculations reveal the reaction mechanism to be mainly a barrierless addition of HO2 to NH2 leading to an intermediate OOH–NH2 (3im1), and then the adduct 3im1 goes through an H transfer forming the product of NH3 and O2. The calculations show that the reaction should be taken place both on the singlet and triplet potential surface. Highlight ► The singlet and triplet potential surfaces of the HO2+NH2 reaction were studied. ► OH and NH2O are the major products on the singlet potential surface. ► But on the triplet potential surface, the products of NH3 and O2 will be dominant. ► The reaction should be taken place both on the singlet and triplet PESs.