Reactivity Controlled Compression Ignition (RCCI) engines are new ideas for decreasing the emissions and fuel consumption in Compression Ignition engines. In this study, numerical study of the combustion and emission characteristics on a RCCI engine with dual fuel natural gas/diesel and methane/diesel are investigated. The numerical simulation is done by Converge software. The natural gas is composition of methane, ethane, propane, carbon dioxide, nitrogen and oxygen. The results were compared with the pressure and heat released in-cylinder, mean gas temperature, emissions and gross power with dual fuel natural gas/diesel and methane/diesel. The results showed that the maximum pressure, mean gas temperature and heat released in-cylinder with methane/diesel are about 5%, 2% and 3% higher than natural gas/diesel, respectively. NOx and Soot emissions with dual fuel natural gas/diesel are about 12.5% and 12% lower than methane/diesel, respectively. Other emissions including unburned hydrocarbon (UHC) and carbon monoxide (CO), was relatively high. Also, the gross engine power for dual fuel methane/diesel was about 4% higher than natural gas/diesel.