The consumption of lubricating oil during the normal operation of an engine is unavoidable, and the consumed lubricating oil can impact the engine’s particulate matter (PM) emissions after participating in the combustion process. This paper conducts research on the changes in the particle size distribution of PM emissions on a direct injection diesel engine when lubricating oil is involved in combustion. By adding a small amount of lubricating oil to the diesel fuel, the study simulates the direct participation of lubricating oil in combustion and its effect on the particle size distribution of PM emissions. The results indicate that after the lubricating oil participates in combustion, it significantly increases the emission of nucleation mode particles, and this effect sharply increases with the higher content of lubricating oil in the diesel. When lubricating oil participates in in-cylinder combustion, the diameter of the primary particles of PM increases, and the impact is more pronounced under low-load conditions.