Past numerical studies on natural gas HCCI engines have provided few thorough analyses of the effects of operating parameters on nitrogen oxides (referring to the mixture of nitric oxide and nitrogen dioxide). A single zone thermodynamic model with detailed chemical kinetics was used to determine the effect of operating parameters on nitrogen oxides emissions. The model employed Chemkin and used chemical kinetics for natural gas with 53 species and 325 reactions. The simulation was conducted for a modified 0.41 single cylinder engine, which possessed a compression ratio of 21.5:1, and had a bore and stroke of 86 and 75 mm, respectively. Several sets of parametric studies were completed to investigate the effect of engine load (imep = 200-600 kPa), speed (600-3000 RPM), equivalence ratio (0.3-1.0), EGR level (0-40%), temperature at IVC (390-460 K), and fuel compositions (0-10% C2H6) on nitrogen oxides emissions. Contributions of different NOx mechanisms have been examined, and the thermal mechanism accounts for over 70% of the total NOx at most of the conditions. The results show significant changes in nitrogen oxides concentrations with varying engine operating conditions. These changes are shown to be strongly dependent on the chemical kinetics and the resulting differences in gas temperature profiles. For this particular study, 50% reduction in nitrogen oxides emissions could result from a load (imep) decrease from 300 kPa to 200 kPa, or an EGR level increase from 0% to 20%. (C) 2011 Elsevier Ltd. All rights reserved.