This study investigated the homogeneous formation and reduction of NO from the oxidation of CH4/NH3 to understand the effect of the high concentrations of H2O on NO emissions. The experimental results about the effects of excess oxygen ratio, gas compositions including H2O, CO2, NH3 and recycled NO, on NO formation and reduction were presented. The mechanism about the effects of high concentration of H2O and combined effect of H2O/CO2 on NO formation and reduction has been discussed. The results indicates that NO formation was markedly enhanced at high concentrations of H2O, which were mainly attributed to the increased OH radicals with steam addition at oxygen-rich conditions. By contrast, the radicals were consumed by reduction radicals, such as CO, H, and CHi, radicals, under oxygen-lean conditions. NO concentrations followed the sequence of H2O/Ar > H2O/CO2 > CO2/Ar under both the oxygen-rich and oxygen-lean conditions, which meant that the highest and lowest NO emissions were recorded in Ar/H2O and CO2/Ar respectively. The combined H2O/CO2 concentration accordingly exerted an effective impact on NO reduction. The results about the effect of NH(3 )concentration in fuel on NO emission showed that NO emission increased with increasing NH3 concentration. Furthermore, NO reburning results implied that with increasing NO concentration, the total NO reduction increased, whereas the reduction ratio decreased.