Nitrogen oxides (NOx) are among the most pollutant species resulting from the combustion phenomena. Unfortunately, all the available measures to alleviate the formation and emission of NOx inhibit the combustion efficiency. Therefore, accurate estimation of NOx in combustion phenomena is an important factor in designing the combustion chambers. The present study addresses this issue. It starts with a comprehensive survey on different physical mechanisms of formation and emission of NOx. Then, a thorough study on mathematical models of the aforementioned mechanisms is presented. Based on the limitations in predicting reactive species from the fluent code, a traort equation for the main NOx constituent is presented. Several sub-models are presented and discussed to express the source term in this traort equation. The turbulence-chemistry interaction is taken into account via a presumed PDF of the mixture fraction and EDC method. A post-processing computer program with several options of the NOx models is developed. The validity of NOx models and the correctness of the program have been assessed by simulating several diffusion flames, and comparing the results with the available experimental data. Finally, prediction of NOx is run for an industrial gas turbine combustion champers,