Cu/SiO2 nanocomposite was synthesized by sol-gel method. The size, morphology and phase structure of as-prepared Cu/SiO2 nanocomposite were analyzed by means of X-ray diffraction and transmission electron microscopy, and its ultraviolet-visible light spectrum was measured in relation to surface plasmon excitation of Cu particles. The tribological properties of as-synthesized Cu/SiO2 nanocomposite as an additive in distilled water were investigated with a four-ball machine, and the morphology and elemental composition of worn steel surfaces were examined with a scanning electron microscope and an X-ray photoelectron spectroscope. Results show that as-synthesized Cu/SiO2 nanocomposite as a lubricant additive is able to significantly improve the tribological properties of distilled water. A protective and lubricious film composed of Cu and a small amount of FeS, FeSO4 and SiO2 is formed on steel sliding surfaces lubricated by distilled water containing Cu/SiO2 nanocomposite. During friction process Cu nanoparticles can be released from Cu/SiO2 nanocomposite to fill up micro-pits and grooves of steel sliding surfaces, resulting in greatly reduced friction and wear of steel frictional pair via self-repairing. The state and thickness of the film formed on the worn surface is closely related to applied load; and Cu/SiO2 nanocomposite might be a promising water-based lubricant additive for steel-steel contact subjected to moderate load. (C) 2012 Elsevier B. V. All rights reserved.