The mechanism of detailed chemical reactions in combustion is very complicated, especially when the formation of pollutant such as NO, is considered. Based on Glarborg＇s and Bilger＇s full model (Glarborg P. et al., Combustion and Flame, 65, 177; (1986), Bilger R. W., Starner S. H., Kee, R. J., Combustion and Flame, 80, 135 (1990)), a simplified model of the mechanism of NOx formation in a CH4/Air system is presented. Sensitivity calculation and eigenvalue-eigenvector analysis (principal component analysis (Vajda S., Valko P., Turanyi T., Int. J. Chemical Kinetics, 17: 55 (1985)) are used in the model. Compared with the conventional methods of simplification of chemical reaction mechanisms such as quasi-steady-state approximation, the eigenvalue-eigenvector analysis can indicate the important reactions efficiently without providing any balance hypothesis. The new model consists of only 15 elementary reactions, which is simpler than the full model composing 39 reactions. The deviations in NOx concentration and other major species between the simplified and the full model are rather small over a wide range of fuel-air ratio. Meanwhile, the calculated NOx concentrations by the simplified model are in quite good agreement with Bartok＇s experimental data (Bartok W. et al., AICHE Symp. Ser. 126 (1972)), especially in the fuel-lean region.