RCCI (reactivity controlled compression ignition) has drawn much attention since it was proposed, as it could lead to high performance and clean combustion. The original concept of an RCCI engine is to use gasoline and diesel as the low and high reactivity fuels, respectively. In this study, biodiesel is considered as a substitute of diesel. Hence, research was numerically conducted on a gasoline/biodiesel fueled RCCI engine. To simulate the combustion process, coupled KIVA4-CHEMKIN was used. The gasoline/biodiesel reaction mechanism as implemented in the code was developed by the authors. Two major factors, namely gasoline to total input energy ratio and SOI (start of injection) timing, have been considered. The varied gasoline energy ratios were applied to both C-SOI (conventional SOI) and A-SOI (advanced SOI) timings. Comparing the combustion characteristics between two SOI timings with varied gasoline energy ratios, it is found that A-SOI could offer more controllability on start of combustion, combustion phasing etc. when varying gasoline ratio. As for the emissions formation, increases in gasoline could reduce NOx and soot emissions simultaneously by achieving more homogeneous combustion. However, the designed spray angle of 78 for C-SOI, when used with A-SOI, would increase the soot formation especially when more biodiesel is injected. (C) 2015 Elsevier Ltd. All rights reserved.