Under lean stratified combustion, differed from the stoichiometric homogeneous charge combustion, flame could propagate through extremely rich air-fuel mixture. 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 speculated with visualized combustion data. 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. To mitigate these emission, effort to accomplish with more homogeneous, but not overly leaned mixture formation in normal butane combustion.