Copper oxide/reduced graphene oxide (CuO/rGO) nanocomposite was synthesized by an effective approach combining supercritical hydrolysis and facile thermolysis. The as-synthesized CuO/rGO exhibited a unique microstructure in which ultrafine CuO nanoparticles were encapsulated by several stacked rGO nanosheets. The highly dispersed CuO nanoparticles on the rGO nanosheets were in spherical shape with diameters around 6 nm and had a base-centered monoclinic crystal structure. Tribological performances of the CuO/rGO nanocomposite as a lubricant additive of 10w40 engine oil were evaluated by a ball-on-disc tribotester under boundary lubrication. As compared to the bare 10w40 engine oil, the engine oil with 0.06-0.18 wt % CuO/rGO exhibits the reductions of 46.62 and 77.05% for friction coefficient and wear rate, respectively. In addition, CuO/rGO as a nanoadditive displays superior lubricating abilities than rGO, CuO, and their mechanical mixture. This synergistic lubricating effect of the CuO/rGO nanocomposite was mainly contributed to its special nanostructure rather than only the coexistence of CuO and rGO. The antiwear and friction-reducing mechanism of the CuO/rGO nanoadditive mainly results from its synergistic lubricating effect and the formed tribofilms on the wear surface due to the spontaneous deposition behavior of the nanocomposite. In addition, influences of the nanoadditives including rGO, CuO, the mixture, and CuO/rGO on the rheological behaviors and the elastohydrodynamic lubrication (EHL) oil film of the modified engine oil were also investigated. Results show that these nanoadditives can increase the viscosity of the engine oil to a certain extent throughout the test temperature range of 0-60 degrees C. The remarkable increase in viscosity by addition of the CuO/rGO nanoadditive corresponds to the thickening of the EHL film and the strengthening of the EHL state for the engine oil with CuO/rGO as compared to the bare one.