Experimental investigations were conducted on a 4-cylinder natural-aspirated direct-injection diesel engine with naturally aspirated hydrogen, focusing on the effects of hydrogen addition on the physicochemical properties of the diesel particulate. Diesel particulates were sampled for off-line analysis, with the aid of TEM and TGA facilities. Hydrogen addition promotes particle oxidation at low engine load and speed due to the increase of exhaust temperature, resulting in smaller particles, but it inhibits particle oxidation at high engine load due to the competition of oxygen between hydrogen and diesel fuel which results in larger primary particles. The replacement of injected diesel fuel by hydrogen inhibits the formation of soot nuclei and decreases its volume density, hence reduces the size of aggregate particles which are more spherical as indicated by an increase of fractal dimension and a decrease of radius of gyration. With increase of engine load, primary particles exhibit more graphitic structure, changing from "onion like" to "shell-core" structure. Hydrogen addition promotes and inhibits primary particle oxidation at low and high engine loads, respectively, and the corresponding primary particles are "turbostratic interlayer" and "shell-amorphous" in structure, respectively. The results of recognized fringe length, tortuosity and fringe separation distance are consistent with the observed morphology. The oxidation reactivity is related to equivalence ratio, being higher at low engine load and speed, which is indicated by the variation of activation energy and ignition temperature. The oxidation reactivity is validated to be related to the nanostructure of primary particles. (C) 2015 Elsevier Ltd. All rights reserved.