The characteristics of exhausted smoke of a methanol DI diesel engine which is ignited by diesel fuel are investigated to clarify the soot formation process. At this engine, very little smoke is exhausted when diesel fuel is kept below a certain amount, so soot and smoke emitting characteristics are studied under the various diesel fuel amounts.By analyzing microstructure of soot, it is found that the soot emitted from the methanol diesel engine is composed of inner core and outer shell, similar to that of the conventional diesel engines. From more detailed qualitative analysis, the calcium percentage from the lubricating oil in outer shell is much higher than that of the conventional diesel engines. In consideration of soot characteristics, spray structure and combustion characteristics, the soot formation process of the methanol diesel engine was clarified. Furthermore, it has become apparent that, in order to reduce soot in the cylinder of engine, oxidizing of the inner core of soot, before the complete soot is formed, is important.The exhausted emissions from the diesel engines for vehicles are becoming one of the urgent subjects to be solved. Especially, smokeless diesel is strongly demanded for the heavy traffic conditions. To reduce smoke drastically, the basic researches on the soot emitted from the diesel engines had been carried out and reported about the soot formation process. (1), (2) and (3) However, most of investigations are discussed from the view points of physicochemical reactions, and few investigated by soot structure analysis. On the other hand, it is known that the major part of soot is an insoluble substance with high carbon density and its shape is very refined. Therefore it is considered that the structure analysis of carbon solid refined particle contributes to the clarification of soot formation and growth process. From this point of view, the investigation by electron microscope, which enables nano order atomic structure and element analysis, is essential. As a basic research to reduce soot emitted from the diesel engines, authors have investigated the soot from direct and indirect injection diesel engines. (4) (5) In these investigations, soot formation and growth process were discussed in relation of combustion condition and physicochemical properties of the soot.