Reactivity controlled compression ignition (RCCI) was investigated on a light-duty optical engine under different fuel stratification degrees, using multiple laser diagnostic techniques. The engine was run at a speed of 1200 rpm and under a load of about 7 bar gross IMEP. To form different fuel stratification degrees, the direct-injection timings of n-heptane were changed, while the port-injection timings of iso-octane was kept constant. The fuel/air equivalence ratio and primary reference fuel (PRF) number were quantified by the fuel-tracer planar laser-induced fluorescence (PLIF) under non-combusting condition. The results indicated that with retarding n-heptane injection timing from -90 degrees CA ATDC (RCCI-90 case) to -10 degrees CA ATDC (RCCI-10 case), regions of higher fuel concentration and reactivity moved downstream to the edge of combustion chamber before high-temperature heat release (HTHR) phase. Timeresolved natural combustion luminosity imaging and single-shot OH PLIF imaging indicated that RCCI-10 case presented a staged combustion process that an auto-ignition first happened in the region of high reactivity around the combustion chamber and then another auto-ignition process took place in the region of low reactivity in the central part of the combustion chamber. The staged combustion feature involved in RCCI combustion could result in lower combustion pressure-rise rate. PLIF images of formaldehyde showed that formaldehyde first formed during low-temperature heat release (LTHR) phase in the regions where n-heptane resided. With retarding n-heptane injection timings, both formaldehyde and OH PLIF images presented more stratified distribution, and the consumption of formaldehyde and formation of OH processes got slower. OH PLIF images indicated that HTHR phase of RCCI could extend to the central part of combustion chamber. In the low-load LTC conceptual model proposed by Musculus et al. (2013), no HTHR happened and UHC formed in the central part of combustion chamber. This meant that RCCI could have less UHC emission than LTC in theory. (C) 2017 Elsevier Ltd. All rights reserved.