Effect of base oil on the characteristics of emitted particulate matter from a four-cylinder turbocharged diesel engine was investigated. This engine was lubricated with two kinds of base oil (characterized as 150 and 350). The viscosity and flash point of 350 base oil are higher than 150 base oil's. Transmission Electron Microscope, Raman spectroscopy and Thermogravimetric analyzer (TGA) were employed to analyze the particle diameter distribution, nanostructure, graphitization degree and its oxidation characteristics. The primary particles diameter distribution conformed to Gaussian distribution, whereas the mean diameter of 150 base oil was larger than 350 base oil's at 1200 rpm and 2000 rpm. The layer fringe length of 350 base oil was shorter than 150 base oil's, while its separation distance and fringe tortuosity were larger. As to the graphitization degree of 150 base oil, the ratio of the D 1 band to G band areas (A D1 /A G