The effect of H2O reactivity on CH4 oxidation in O2/H2O combustion was studied using the reactive molecular dynamics method.The results obtained under fuel-rich condition showed that replacing N2 gas with H2O inhibit the oxidation rate of CH4 at low temperatures due to the equilibrium reasons and the third body efficiency of H2O.However,the presence of H2O advanced the oxidation of CH4 because of H2O reactivity at high temperatures.The amount of OH radicals and H2 molecules in fuel-rich O2/H2O combustion was obviously greater than that in O2/N2 combustion which proved that H2O molecules mainly take part in the reactions through H2O+H → H2+OH and H2O+O → OH+OH.