Emissions of nitrogen oxide (NO) from combustion are modeled based on the constrained free energy method (CFE). The CFE method is an extension of the Gibbs' free energy minimization technique, using additional immaterial constraints which allow for the calculation of local thermodynamic equilibria. Chemical reactions, enthalpic effect, and thermochemical state variables are calculated concurrently. Various kinetic constraints were applied in this study for modeling thermal and fuel-based NO emissions for a number of combustion problems. The results were validated against data obtained from detailed kinetic models. The results indicate that the CFE method can be used for modeling post-flame thermal and fuel-based NO emissions. Where more specific estimation of radical buildup, radical overshoot, and ignition delay is needed, however, the accuracy of the applied CFE method was deemed insufficient.