Copper nanoparticles with different average particle sizes of 61-139 nm were facilely prepared by the wet chemical reduction of copper hydroxide without or with the use of organic modifier. Sodium dodecyl benzene sulfonate as an organic modifier effectively decreased the sizes of copper nanoparticles as compared to sodium citrate. The presence of polyvinylpyrrolidone favored the formation of the small-sized copper nanoparticle. The tribological properties of lithium-based complex grease were effectively improved by addition of copper nanoparticles (≤2 wt%). The improvement of grease tribological properties could be attributed to deposition of soft copper nanoparticles and tiny bearing action on rubbing surfaces.