The experiment and kinetic modeling prediction were conducted to investigate the effect of H2O vapor on NO reduction by CO. The experiment was performed in a 36-kW combustion reactor, and modeling prediction was done using the plug-flow reactor model. In the presence of H2O, the formation of free-radical H is conducive to NO reduction. H2O has a complex effect on NO emission: at T < 1100 degrees C, NO emission decreases with the increase of H2O content and temperature, the reaction pathway of NO reduction is NO -> HNO -> NH -> N2O -> N-2, and the controlling factors of NO reduction are temperature and H2O concentration; at 1100 degrees C < T < 1400 degrees C, NO emission decreases as the H2O content increases and remains constant with the change of temperature at [H2O] < 1.0%, NO reduction depends on H2O concentration rather than temperature, and increasing H2O concentration makes free-radical H reach high equilibrium concentration; at T > 1400 degrees C, the effects of H2O content and temperature on NO emission at [H2O] < 1.0% are similar to that at T < 1100 degrees C, NO emission increases with H2O content but decreases with temperature at [H2O] >1%, NO reduction depends on the combined effects of H and CO.