Abstract: SiO_2/SnO_2 complex nanoparticles were prepared making use of a chemical method. The tribological properties of the resulting complex nanoparticles as an additive in 500SN oil were evaluated using a four-ball machine, while the self-healing action of the additive to the worn steel surface was examined on a ring-on-block test rig. The morphologies and elemental distributions of the worn steel surfaces were analyzed by means of scanning electron microscopy and energy dispersive X-ray analysis. It was found that the complex nanoparticles as the additive contributed to greatly improve the antiwear ability and load-carrying capacity of the base oil, but they had little effect on the friction-reducing ability of the base stock at a relatively small volume fraction. Moreover, the complex nanoparticles were capable of self-healing the worn steel surface at proper test conditions. The improved antiwear ability and load-carrying capacity of the base stock and the self-healing action to the worn steel surface in the presence of the complex nanoparticles as the additive were attributed to the adsorption and deposition of the nanoparticles on the sliding surface and the subsequent formation of a boundary lubricating layer with a small shearing strength.