4. The tribological properties of four kinds of monolayer protected nanoclusters named oil-soluble nano-copper, oil-soluble nano-titania, oil-soluble nano-silver, oil-soluble nano-lanthanum fluoride were evaluated. The results show that they all have unique properties such as anti-wear, friction-reduction, anti-oxidation, load-carrying capacity. The action mechanisms of the coated nanoparticles as lubricant additives, based on the surface analysis results, was briefed as: combining the friction