The mechanism for the NCO + NO reaction has been studied using a high-level ab initio molecular orbital G2M method in conjunction with variational RRKM calculations. The results indicate that the reaction occurs primarily via the following singlet potential energy surface: (1) NCO + NO ⇌ OCNNO → N2O + CO; (2) NCO + NO ⇌ OCNNO → c-OC(O)NN- → N2 + CO2. Cis-OCNNO is the main intermediate to form these products. The decomposition of cis-OCNNO to the products N2O + CO is energetically less favorable than the cyclization process forming c-OC(O)NN- by 8.2 kcal/mol, but both processes take place via tight transition states. These tight transition states coupling with the loose association channel give rise to the experimentally observed strong negative temperature dependence. Calculated results also indicate that the total and individual rate constants have a strong angular-momentum dependence. The calculated values are in good agreement with experimental data both for the rate constant and product branching ratios. These results are qualitatively consistent with those predicted previously by the conventional RRKM calculations based on the energetics computed at the BAC-MP4 level of theory.