A density function theory study is presented for the HNO reversible arrow HON isomerization assisted by m water molecules (m = 1-4) on the singlet state potential energy surface. Two modes are considered to model the catalytic effect of m water molecules: (1) water molecule(s) directly participate in forming a proton transfer loop with HNO/HON species and (2) water molecules are out-of-loop, modeling the outer-sphere water effect from the other water molecules directly H-bonded to the loop (referred to as out-of-loop waters). Two mechanisms are proposed for the mono- water- assisting isomerizations and one for each of the other cases. The reactant and product of all groups have been characterized for all potential energy surfaces. For the monohydration mechanism, the reactant complex is connected to the product complex via two determined saddle points, and the reaction heat is 35.5 kcal.mol(-1) at the B3PW91/6-311++G** level. The corresponding forward/backward barrier lowerings are obtained as 7.4/1.2 (AT2) and 34.1/28.3 (AT1) kcal.mol(-1), respectively, compared with the no-water-assisting isomerization barrier T (72.6/31.3 kcal.mol(-1)). When one to three out-of-loop water molecules are considered, their effects on the three energies are small, and the deviations are not more than 2 kcal.mol(-1) compared with the original monohydration assisting case (AT1). For the dihydration assisting mechanism, the reaction heat is 30.5 kcal-mol(-1), and the forward/backward barrier lowerings become 41.7/ 30.9 kcal.mol(-1) compared to T. Further increasing by 1-2 out-of-loop waters does not obviously change the energetics. For trihydration, the forward/backward barriers further decrease as 43.5/30.9kcal.mol(-1), and the reaction heat decreases by 12.4 kcal-mol(-1). The same is true for the influence of the additional out-of-loop waters. However, when four water molecules are involved in the reactant loop, the corresponding energy aspect increases slightly. The forward/backward barrier lowerings for the tetrahydration mechanism become 41.8/29.4 kcal.mol(-1), respectively, smaller by 1.7 and 1.5 kcal.mol(-1) than the trihydration situation. Therefore, it can be concluded that the most favorable hydration-assisting mode should be one with three in-loop waters. Such hydration-assisting isomerization pathways can exist in water-dominated environments, for example, in the organism, and are significant to energy transferring.