This paper presents the effects of N-2, O-2 and Ar in excess air on combustion and NOx emissions of a natural gas spark-ignition engine. Based on Arrhenius Law and Zeldovich Mechanism, seven different kinds of effects were defined as well as their quantitative indicators. Excess air and N-2 were separately added into the intake charge with fuel remaining unchanged in a 6-cylinder turbocharged natural gas engine. The results show that the essential reason why NOx emissions vary with increasing excess air ratio (lambda) is that the oxidation effect of O-2 varies with the in-cylinder temperature, which is controlled by the thermal effect of excess air. The contributions of N-2, O-2 and Ar on the thermal effect are 82.6-86.1%, 13.3-16.7% and 0.6-0.7%, respectively. The chemical effect of O-2 is fully offset by the dilution effect of N2 and Ar. The contributions of N-2 and Ar on the dilution effect are 98.31% and 1.69%, respectively. The increase of NOx emissions caused by O-2 is found to be increased first and then decreased with increasing lambda. From the view of the composition of air, the high level of N-2 is the main reason why NOx emissions decrease significantly with increasing lambda. (C) 2015 Elsevier Ltd. All rights reserved.