Abstract Compression ignition engines have always been attractive due to high thermal efficiency. Advanced combustion modes have been developed for internal combustion engines to reduce both harmful emissions and fuel consumption. One of these technologies is the reactivity controlled compression ignition (RCCI) mode. Mixture formation in the RCCI engine is provided by a low reactivity fuel injection (port fuel injection) during the intake stroke and stratified high reactivity fuel injection into the cylinder (direct injection) during the compression stroke. In the present study, the effect of reforming gases on RCCI combustion was investigated numerically. The RCCI experimental data were obtained from a previous study performed on a 1.9-liter GM brand gasoline engine fueled with gasoline/diesel fuels. The simulation was performed via Converge Computational Fluid Dynamics (CFD) code, and numerical results were validated with the experimental data. The maximum in-cylinder pressure was recorded as 6.84 MPa with nitrogen addition. It is reached up to 15.12 MPa in case of using hydrogen and nitrogen together. Soot production reached a maximum level of 8.5 × 10 - 3 g/kg-fuel with 72% nitrogen addition. However, soot pollutants were reduced via 4% hydrogen substitution and recorded as 2.3 × 10 - 5 g/kg-fuel. While the in-cylinder gas temperature was 1295 K with 72% nitrogen, it reached 3578.5 K with hydrogen addition.