The effect of diluent addition on NOx formation in a laminar CH4/air coflow diffusion flame was investigated by numerical simulation with experimental verification. The hydrocarbon fuel stream was diluted with N2, CO2, and Ar. The volume fraction of diluents systematically changed from 0.0 to 0.5. The simulation data agree well with the experimental one. The computational results indicate that overall the three diluents reduce the formation of NO and the effects vary from weak to strong in the order: N2, Ar and CO2. Differences between the influences of the various diluents are discussed in terms of the thermal, the dilution and the direct chemical effects, respectively. Further, the addition of CO2 reduces the formation of NO2, while the addition of N2 or Ar has little effect on it. However, the formation rate of N2O increases by each of the added diluents.