In this paper, a numerical study is carried out to investigate the combustion and emission characteristics of reactivity controlled compression ignition (RCCI) combustion mode in a heavy-duty, single-cylinder diesel engine with gasoline and diesel fuels using KIVA-CHEMKIN code with a reduced primary reference fuel (PRF) mechanism. Firstly, a comparison is performed between RCCI and CDC performance and emissions to show the superior characteristics of RCCI combustion. Then, the effect of diesel fuel mass fraction in SOI-1 on combustion and emissions of RCCI engine is studied. It is shown that by increasing the diesel mass fraction in SOI-1, combustion event occurs earlier and PPRR is slightly higher. But this parameter has a trivial influence compared to PRF number and SOI timing.