Lean stratified combustion is a mean to dilute the fuel-air mixture leaner than stoichiometric ratio, by using stratification of fuel gradient in a spark ignition engine. Under the lean stratified combustion, differed from the stoichiometric homogeneous charge combustion, flame could propagate through extremely rich air-fuel mixture, while the global air-fuel mixture is under lean condition. The rich mixture causes considerable amount of particulate matter, but, due to large effect of efficiency improvement, the attractive point is on fuel economy compare to homogeneous charge SI combustion. The easiest way to reduce particulate matter is changing fuel to gaseous hydrocarbon, to minimize evaporating and mixing period. In this study, to reduce the particulate emission and to develop the way to mitigation of emission, the emission data of particulate under low and medium-low load conditions from normal butane fueled research engine are dealt to optimize combustion strategies, with respect to injection and ignition. Especially, particulate number density were collected in the research engine, and the causes of particulate formation were studied with visualized combustion data. Through visualization data, the sources of particulate emission were collected through sooting flame position, and the characteristics. In the results of analysis, large amount of small particle source in combustion could be caused even in longer mixture formation time, which the more homogeneous mixture could constructed. These results were caused by multi-point diffusion burn phenomena in burned gas area, which was not observed in shorter timing gap from injection to ignition. The number emission were enhanced in appropriate time gap with higher work conversion efficiency, additionally with higher in-cylinder temperature circumstance. Faster combustion in the case caused more wetting and sooting flame on components such as injector and spark plug, in addition to sooting flame before the flame let. In addition, the particle size distribution were shift toward larger size with the sooting flame, and the numbers also were increased. To improve these emission, effort to accomplish with more momentum exchange in addition to evaporation. Nevertheless, the overly leaned mixture would deteriorate efficiency of the combustion and flame propagation, accompanying hydrocarbon and carbon-monoxide increase. With sweeping the spark and injection timing, the optimum for efficient and clean combustion could be found with retarded injection timings.