Microscopic analyses of the effects of ash on particulate matter oxidation are rather scarce. In this study, three different lubricating oils with varying ash contents were used to investigate their effects on the nanostructure of diesel particulate matter. The nanostructure and nanostructure parameters, including fringe length, fringe separation distance, and fringe tortuosity, were studied using high-resolution transmission electron microscopy. The results show that all samples obtained from blending with different lubricant oil present typical core–shell structures. The inner cores remain relatively unchanged, whereas the thickness of the outer shells increases with the increasing ash content in the lubricant oil under the same working conditions. The fringe length increases and the fringe separation distance decreases with the rising ash content in the lubricant oil operating in the same working conditions. The fringe tortuosity decreases when the ash content in the lubricant oil increases from 0.92% to 1.21%, but shows little change when the ash content in the lubricant oil increases from 1.21% to 1.92%. Based on the effects of ash on the nanostructure parameters, it can be inferred that the oxidation activity of particles decreases with increasing ash content in the lubricant oil.