The effect of different diluents on the NO

x

emission of the combustion of natural gas with CO

2

and N

2

was studied with premixed methane/air/diluent flame. The chemical effect and thermal effect of CO

2

on the NO production was revealed via conducting reactionpath analysis. The numerical results show that: 1) CO

2

is more effective than NO in emission reduction. The chemical effect is achieved by consuming larger part of CH

2

(s) in the flame zone

leading to smaller NO production subsequent to lower CH concentration; 2) compared to the chemical effect

the thermal one plays a more important role in NO emission by reducing the adiabatic flame temperature of the mixture; 3) the introduced diluents enlarge the ignition delay time slightly and greatly reduce the laminar flame speed of the mixture

indicating that low NO

x

emission characteristic of diluents accompanies with significant inhibiting effect on the combustion of the mixture.