Chemical equilibria involving 10 species and adiabatic reaction temperature of methane combustion in air under various conditions have been calculated in detail by means of total Gibbs energy minimization of the system. The calculation data show that the adiabatic combustion temperature of CH4 and air at stoichiometric ratio is up to about 2200 K, and the equilibrium concentration of NO is about 0.0018, however that of NO2 is only 1 × 10-6. A large amount of carbon deposition emerges when the CH4 concentration is above 26.5%. The NO and NO2 appear only when the CH4 concentration is below 16%.The maximum equilibrium concentrations of NO and NO2 are 0.0028 and 2× 10-6 respectively, at about 8%CH4 concentration. The NO and NO2 concentrations increase with the system temperature at a low CH4 concentration. However, both of them can be decreased when CO2 or steam is introduced into the system, which also decreases the adiabatic combustion temperature. The decrease in adiabatic temperature caused by CO2 addition is more appreciable than that caused by the addition of H2O. Pressure does not have a notable effect on the system equilibrium at a low methane concentration.