Exhaust gas recirculation (EGR) is one of effective measures used in natural gas (NG) engines to reduce nitrogen oxides (NOx) emissions. Each component of EGR gases can exert different effects on NG combustion and NOx formation rates, such as thermal effect, diluent effect, and chemical effect. In this study, the thermal and diluent effects of the main components of EGR gases, including carbon dioxide (CO2) and nitrogen (N-2), were experimentally investigated. The experiments were arranged based on an electronically controlled heavy-duty natural gas spark-ignition (SI) engine with multi-point injection and 6-cylinder. In order to define the diluent effect of EGR components, argon (Ar) was introduced to the test, for its low and unchangeable specific heat capacity under different temperatures. The results showed that the contribution of the diluent effect on NOx reductions was 50-60% and 41-53% for N-2 and CO2 respectively, and the relevant contribution of the thermal effect was 40-50% and 47-59% respectively. CO2 had greater effects on NG combustion and NOx formation rates than N-2 at the same dilution ratio. Increasing Ar improved the thermal efficiency due to the higher specific heat ratio provided. Meanwhile, NOx emissions were found to be decreased with increasing DR as a result of the diluent effect of Ar on combustion and NOx formations. (C) 2014 Elsevier Ltd. All rights reserved.