Abstract: Diamond nanoparticles were prepared by detonation of explosives with negative oxygen balance. The effect of the diamond nanoparticles as additive on the tribological behavior of 15W/30 engine oil was examined by means of bench test of cylinder liner/piston ring and ball-on-disc friction and wear test, respectively. The action mechanism of the diamond nanoparticles is discussed as well. As the results, the diamond nanoparticle as additive has little effect on the physical-chemical properties of the base stock but improves the tribological behavior of the base stock considerably. Namely, the load-carrying capacity and friction-reducing and antiwear ability of the 15W/30 engine oil were greatly increased by the addition of the diamond nanoparticles. This could be attributed to the formation of a boundary lubricating film containing diamond nanoparticles on the rubbing surface. In other words, the diamond nanoparticles act to increase the load-carrying capacity and decrease the wear of the frictional pair considerably, because they have high hardness and are liable to deposit on the rubbing surface owing to their small-scale effect. Such a kind of wear-resistant and load-carrying capacity of the diamond nanoparticles as additive could be confirmed by the increase of the worn surface hardness.