The effect of H2O addition on the oxidation of methane and ammonia during oxy-fuel combustion was investigated both experimentally and numerically. Comparison experiments between O-2/CO2 and O-2/CO2/H2O atmospheres were conducted in a flow reactor at atmospheric pressure with equivalence ratios ranging from fuel-rich to fuel-lean and temperature from 973 to 1773 K. The experimental results indicate that the effects of H2O addition shift the onset temperature of oxidation to the lower values, inhibit CO formation significantly, and enhance NO formation remarkably. The chemical kinetic mechanism, which was hierarchically structured and updated in our previous work, captured the main characteristics of CO and NO formation satisfactorily. The presence of H2O leads to far higher OH radical concentrations in the CO2/H2O atmospheres. The ultrahigh OH radical concentrations dramatically enhance the reactions between OH and amine radicals, resulting in the significant enhancement of pathways NH2 -> NH -> HNO -> NO and NH2 -> NH -> N -> NO in CO2/H2O atmospheres. Meanwhile, NH2 -> CHxNHy/HNCO -> NCO -> NO is vastly demoted in CO2/H2O atmospheres. The increase in pathways NH2 -> NH -> HNO -> NO and NH2 -> NH -> N -> NO is always much more than the decline in pathway NH2 -> CHxNHy/HNCO -> NCO -> NO. Hence, H2O addition in oxy-fuel combustion enhances NO formation during the oxidation of methane and ammonia. In addition, the effects of H2O addition become stronger on enhancing NO formation with the increasing H2O concentration in CO2/H2O atmospheres by further amplifying the amount of OH radicals.