Main purpose of this paper is investigating on the effect of reformer gas addition on combustion, performance and emission characteristics of a n-heptane fueled HCCI supercharged engine. A validated multi zone model, which is accurate for modeling of HCCI engine, is used. Heat and mass transfer between zones and convective heat transfer between in-cylinder charge and combustion chamber walls are considered in the multi zone model. A semi detailed chemical kinetics mechanism, containing 57 species and 290 reactions, is used for simulating the combustion process of n-heptane. Four different values of reformer gas are added to the main fuel and its effects on engine performance and chemical reactions are studied. The results show that addition of reformer gas retards the start of combustion and causes to lower in cylinder peak pressure and temperature. Chemical, dilution and thermodynamic effects of RG are studied and the results show that the chemical effect of reformer gas is more significant than its dilution and thermal effects. Hydrogen in comparison to carbon monoxide has more chemical effects. Chemical analyses show that RG affects the chemical reactions and intermediate species concentration. (C) 2017 Elsevier Ltd. All rights reserved.