One-dimensional premixed freely-propagating flames for (CH4+CO2/H2O)/air(79%N-2+21% 02) mixtures were modeled using ChemkinII/Premix Code with the detailed mechanism GRI-Mech 3.0. The investigation of the effects of CO2 and steam addition on the H-2 intermediate formation and NO emission was conducted at the initial conditions of 1 atm and 398 K. Both physical and chemical effects of CO2, H2O on laminar burning velocities and adiabatic flame temperatures were also analyzed. The calculations show that with the increase of proportional to CO2 and proportional to H2O, both physical and chemical effects of CO2 and H2O result in the reduction of laminar burning velocities (LBVs) and adiabatic flame temperatures (AFTs) in which the chemical effects of CO2 addition are more significantly than H2O. Especially, the chemical effects of steam promote the increase of AFTs and the influence in rich BG65 flames are larger than in methane. With a proper amount of H2O addition, the chemical effects of H2O on the peak concentration of H-2 are more significantly than physical at Phi = 1.2. Moreover, CO2, steam and their mixture addition have significant reduction on the NO emission. The most sensitive reaction for the formation of H-2 and NO emission were determined. The responsible reactions for H-2 formation and NO emission are R84 OH + H-2 double left right arrow H + H2O and R240 CH + N-2 double left right arrow HCN + N (a prompt routine), respectively. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.