Legislation limiting NOx emissions from natural gas engines dictate a need for greater understanding of the mechanisms governing its formation, and more accurate models for predicting its dependence on the engine design and operating parameters. This work concerns the application of a one-dimensional, multi-zone, thermochemical model for calculating NO formations from a small bore, lean burning, natural gas engine. Because of the potential for significant non equilibrium behavior due to lean operation, high engine speeds and small combustion chamber sizes, detailed chemical kinetic mechanisms are used to integrate the reaction rates over the duration of the product combustion history. The model is used to predict NO emission at two lean conditions: ϕ = 0.91 and ϕ = 0.66. Results show good agreement with the experimental data, with better agreement for the higher equivalence ratio case for conditions which are investigated in detail and are related to the mechanism governing the net formation rate in each case. We show that the richer mixture produces NO by processes which are formation limited in the colder, later burned region, and decomposition limited in the hotter, earlier burned region. In the lean engine, however, NO is formed solely via a formation limited process. The results suggest several avenues to reducing both the absolute value of NO concentration in the exhaust by changing the design and/or operating parameters, as well as the specific NO formation.