Five different lubricating oils with three different sulfur contents (0.182%, 0.583%, and 1.060%) and three different ash contents (0.48%, 1.21%, and 1.99%) were used in this study to investigate their effects on the morphology of diesel particulate matter (PM). The aggregate morphology and nanostructure of primary particles in diesel PM were analyzed using transmission electron microscopy. The primary morphological parameters were determined using an advanced image-processing method. The results show that, with increasing sulfur content and ash content, the macroscopic structure of the primary particles can transform from chainlike to more complex and disordered agglomerated structures, whereas the inner cores change little, the shell thickness decreases, and the thickness of the amorphous materials attached to the outer shell increases. The fractal dimension (D-f) of the PM aggregate morphology varies from 1.656 to 1.811, and the mean primary-particle diameter (d(p)) of the PM is mainly in the range of 10-30 nm. Although dp decreases slightly when the ash content increases from 1.21% to 1.99%, increasing sulfur and ash contents generally increase both D-f and dp. As such, the effects of the sulfur and ash contents in lubricating oil on dp are greater than their effects on D-f. Regarding the nanostructure of the primary particles, the most frequently observed ranges for the fringe length (L-a), tortuosity (T-f), and fringe separation distance (D-s) are <2.4 nm, 1-1.8, and 0.2-0.4 nm, respectively. Higher sulfur and ash contents lead to smaller L-a and D-s values and higher T-f values. Greater d(p) and D-f values and smaller D-s values reduce the oxidation activity of diesel PM, whereas smaller L-a values and larger T-f values increase the oxidation activity of diesel PM. Based on the relationship between the morphology and the oxidation activity of diesel PM, the findings presented in,this study will be of great value in future work for reducing diesel PM emissions.