The purpose of this study is to investigate soot formation and oxidation behaviors with two combustion systems for a DI Diesel engine.The conventional DI combustion system and the conical spray combustion system (CSCS system) were used in a single-cylinder Diesel engine. Under various operation conditions, the measurements were taken of soot, NOx and oxygen concentration by transient sampling technique; KL factor and flame temperature by the two-color method; particulates emission by dilution sampling mini-tunnel. The soot microstructure was also investigated with transmission electron microscope.The temporal histories of soot concentration and KL factor show that soot concentration in the combustion chamber with CSCS is lower than that with a conventional combustion system. The main reason for this is that the spatial distribution of fuel-air mixture is more uniform with the CSCS.The results from electron micrographic analysis show that soot oxidation is influenced by aggregation and agglomeration of soot particles. A very small amount of crystal soot with an unstable lattice is formed at higher flame temperature.