The reaction of NCN with O2 has been investigated by ab initio molecular orbital and transition state theory calculations. The mechanisms for formation of possible product channels involved in the singlet and triplet potential energy surfaces have been predicted at the highest level of the modified GAUSSIAN-2 (G2M) method, G2M (CCI). The stationary points of species on the low-energy paths were also examined with the third-order Rayleigh-Schrödinger perturbation (CASPT3) and the multireference configuration interaction including Davidson's correction for higher excitations (MRCI + Q) theories at the CASPT3 (4, 4)/aug-cc-PVTZ//CASSCF (4,4)/cc-pVDZ(4,4) and MRCI+Q(4,4)/aug-cc-PVTZ //CASSCF(4,4)/cc-pVDZ(4,4) levels. The rate constants for the two low-energy channels, NCN + O2 → cis,trans-NCNOO → NCO + NO (script k sign1) and NCN + O 2 → cis,trans-NCNOO → CNO + NO (script k sign2), have been calculated in the temperature range of 1000-3000 K based on the energies obtained at the G2M(CC1) level. The results show that the formation of NCO + NO is dominant, and its branching ratio script k sign1|(script k sign1 + script k sign2) = 0.83 is almost temperature independent from 1000 to 3000 K. The total rate constant can be expressed by script k signt = 7.29 × 10-15T0.51 exp(-12370/T) cm3 molecule-1 s-1 in the 1000-3000 K range. Overall, the reaction is quite slow because of the high entrance and exit barriers. Based on the predicted heats of reaction and the known heats of formation of NCN and NO, the heats of formation of NCO and CNO at 0 K have been calculated to be 29.8 and 92.6 kcal/mol, respectively. The former agrees closely with the available experimental value, 30.4 ±1.0 kcal/mol.