A hydrogen fueled internal combustion engine (HICE) CFD simulation model consisting of detailed chemical reaction mechanisms was built using CONVERGE to study the NOx formation mechanisms. The Simulation Results are consistent with the experiments we had reported. The Simulation results show that the temperature inside the flame front and the OH concentration in the flame front increased with the fuel -air equivalence ratio. NO, as the major component of NOx, was generated abundantly during the rapid combustion period with the temperature rising and decreased after the rapid combustion period to a stable amount when the temperature dropped below 2200 K in cylinder. NO was generated mainly through three route named as thermal NO, NNH and N2O. The Thermal NO path contributed a large proportion of the total NO emissions and the contribution increased with the fuel -air equivalence. NNH and N2O routecontributed 24.2% of the total NO emissions when the fuel -air equivalence was 0.6, but contributed -23.9% when the fuel-air equivalence was 1.0. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.