In recent years, strict emission regulations, the environmental awareness, and the high price of conventional fuels have led to the creation of incentive to promote alternative fuels. Among the alternative fuels, natural gas is very promising and highly attractive for its abundant resources, clean nature of combustion and low encouraging prices. But nitrogen oxides (NOx) emissions are still a problem in natural gas engines. In order to reduce NOx emissions, carbon dioxide (CO2), nitrogen (N2) and argon (Ar) were respectively introduced to dilute fuel-air mixtures in the cylinder. To this aim a 6.62 L, 6-cylinder, turbocharged, electronic controlled large-powered NG engine was subjected to a basic performance test to observe the effects of CO2, N2 and Ar on fuel economy and NOx emissions. During the test, the engine speed and torque were separately kept at 1450 r/min and 350 Nm. The results showed that the engine using Ar as dilution gas produced the highest thermal efficiency followed by N2 and then CO2 when NOx emissions were reduced to the same levels. With increasing dilution ratio (DR) of CO2 and N2, the ignition delay and combustion duration were prolonged. On the contrary, no obvious changes of combustion phase were found when using Ar as the dilution gas. THC and CO emissions increased slightly while NOx emissions showed a linear downward trend with increasing DR regardless of the dilution gas species. NOx emissions respectively decreased by 42%, 65% and 80% when using Ar, N2 and CO2 as dilution gases at 12% DR.