The objective of this study is to clarify the particulate composition at each particle size and the influence of engine operating conditions and fuel properties on diesel particulate size distribution. Size distribution of particulate matter emitted from the latest Japanese DI diesel engine was measured using a Cascade Impactor. Most of the particles were smaller than 10-micron aerodynamic diameter and a significant percentage of the particles were shown in the aerodynamic diameter range less than 50nm. The chemical analysis of particles collected at each stage of Cascade Impactor showed that the aerodynamic diameter of the sulfate particle was less than 50nm. A large amount of SOF and SOOT was found in small particle size range, but the percentage of SOF in the large aerodynamic diameter particles was higher than that in the small particles. Moreover, when the percentage of SOF in the total particulate matter became higher by lowering the engine torque, the particles with the small aerodynamic diameter decreased. On the other hand, the influence of the fuel properties, such as distillation characteristics, cetane number, and fuel compositions was small within the limit of usual diesel fuel properties. However, the particles of small aerodynamic diameter increased compared with usual diesel gas oil delivered from mineral oil when the n-paraffin fuel was used.