This study systematically investigates the detailed mechanism of nitrogen oxides (NO.) in CH4 and CH4/H-2 jet flames with O-2/CO2 hot coflow. After comprehensive validation of the modeling by experiments of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147-1154]; the effects of CO2 replacement of N-2, mass fraction of oxygen in the coflow (Y-O2), and mass fraction of hydrogen in the fuel jet (Y-H2) on NO formation and destruction are investigated in detail. For methane oxy-fuel combustion, the NNH route is found to control the NO formation at Y-O2 < 3%, while both NNH and N2O-intermediate routes dominate the NO production at 3% < Y-O2 < 10%. When Y-O2 > 10%, NO is obtained mainly from thermal mechanism. Moreover, in the oxy-combustion of methane and hydrogen fuel blends with Y-O2 = 3%, with hydrogen addition the contribution of the NNH and prompt routes increases, while that of the N2O-intermediate route decreases. Furthermore, the chemical effect of CO2 is significant in reducing NO in both oxy-combustion of methane with Y-O2 < 3% and combustion of methane and hydrogen fuel blends with Y-H2 < 10%. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.