NOx formation is one of the main ecological problems nowadays as nitrogen oxides is one of main reasons of acid rains. Nitrogen oxides (NOx) are formed as a result of oxidation of nitrogen which occurs in the air and in a fuel.The aim of this work is to research the formation of NOx in combustion of gaseous fuels. The ANSYS model was designed according to the calculation to provide full combustion and good mixing of the fuel and air. In this paper we research influence of combustion air composition by simulation combustion with the different N2/O2 air ratio on NOx formation.The theoretical and experiential knowledge allow understanding and predict the mechanisms of NOx formation.