Silica nanospheres with good dispersibility in oily solvents were prepared using a facile in-situ surface-modification route. The microstructure of resultant surface-capped silica nanospheres was analyzed with a transmission electron microscope. The tribological behavior of the silica nanospheres as additive in engine oil SE 15W/40 was evaluated using a four-ball machine; and the morphology and composition of the worn steel surfaces were analyzed using a scanning electron microscope equipped with an energy dispersive X-ray analysis attachment. It was found that organic modifier hexamethyldisilazane (HMDS) was combined with nano-SiO2 by covalent bonds, resulting in varied surface properties and improved dispersibility of the nanoparticles in many organic mediums. Besides, oil-soluble silica nanospheres as additive in the engine oil had excellent friction-reducing and antiwear abilities, showing promising application prospect.