As a zero-carbon fuel and a hydrogen derivative, ammonia is promising for large-scale use in internal combustion engines under the global decarbonization background. Although ammonia fuel itself does not contain elemental carbon and cannot produce carbon dioxide, it contains elemental nitrogen and produces nitrogen oxides (NOX) emissions during combustion. Accordingly, it is essential to understand the formation and evolution of NOX during ammonia oxidation as a prerequisite for finding solutions to control NOX emissions. Since the emission formation is chemically reaction-driven, this paper investigates the ammonia low and high temperature oxidation processes via laminar flame and ideal reactor models, which can provide steady-state NOX formation characteristics to be studied and eliminate unpredictable turbulence and gradients of species concentration and temperature in the engine combustion chamber. Moreover, this study investigates the ammonia combustion process under thermodynamic conditions representative of the engine in-cylinder environment. One challenge in understanding the NOX formation mechanism during ammonia combustion is the coupling of fuel NOX (i.e., nitrogen from ammonia) and thermal NOX (i.e., nitrogen from the atmosphere). The main innovation of this article is the introduction of a methodology to decouple fuel nitrogen and atmospheric nitrogen. The results prove that this method is effective regardless of the operating conditions. In addition, unlike the thermal NOX whose concentration is related to temperature and residence time, fuel NOX, especially nitric oxide (NO) and nitrous oxide (N2O), are important intermediate species and are active in the reaction zone and during ignition. Furthermore, the concentration of fuel NOX and thermal NOX are of comparable order of magnitude and they are sensitive to the combustion boundary conditions (e.g., temperature, equivalence ratio, and hydrogen addition). Specifically, increasing the temperature favors the thermal NOX formation, and fuel-rich operation reduces both fuel NOX and thermal NOX concentrations. Also, mixing ammonia with hydrogen can increase fuel NOX and thermal NOX levels simultaneously. Consequently, the cost of using hydrogen as a combustion promoter to improve the ammonia chemical reactivity is to increase the difficulty of NOX emission control. Overall, all of these findings support the need for further fundamental research on ammonia combustion to accelerate the engine transition to carbon neutrality.