The singlet and triplet potential surfaces of the HO 2 +NH 2 reaction have been investigated at the CCSD (T)//B3LYP/6-311++G (3df, 3pd) level. On the singlet potential surface, the most favorable association way starts with a barrierless addition of the HO 2 to NH 2 leading to HOO–NH 2 ( 1 im2 and 1 im3). The product of OH and NH 2 O should be the major product, which obtained by a direct dissociation of the adduct 1 im3 without an exit barrier. But on the triplet potential surface, DFT calculations reveal the reaction mechanism to be mainly a barrierless addition of HO 2 to NH 2 leading to an intermediate OOH–NH 2 ( 3 im1), and then the adduct 3 im1 goes through an H transfer forming the product of NH 3 and O 2 . The calculations show that the reaction should be taken place both on the singlet and triplet potential surface.