Abstract: In modern industry, the friction parts of mechanical equipment are faced with more demanding conditions, such as higher loads, higher operating speeds, longer operating cycles, higher temperatures, greater temperature differences and more frequent starts and stops, which causes a large loss of energy. Using nanoscale lubricating oil additives is one of the most effective ways to control friction and wear, which is of great significance for energy saving, emission reduction and environmental protection. As a lubricant additive, CeO₂ nanoparticles can not only reduce friction and wear, but also can repair worn area, thereby adding to the service life and safety of mechanical devices. However, CeO₂ nanoparticle exhibits poor friction activity and extreme pressure properties. Therefore, 2-ethylhexanoic acid (EHA) and adipic acid (AA) were selected as the modifiers to prepare surface-capped CeO₂ nanoparticles by in-situ surface modification method. The structures of the as-prepared EHA-capped and EHA/AA-capped CeO₂ nanoparticles were characterized. Their friction-reducing and antiwear abilities as well as extreme pressure properties in AN5 base oil were evaluated, and their tribomechanisms were analyzed. Furthermore, the effect of the surface modifiers on the film-forming ability of the EHA-capped and EHA/AA-capped CeO₂ nanoparticles upon rubbing was discussed. Quartz crystal microbalance (QCM-D) tested the adsorption behavior of EHA-capped and EHA/AA-capped CeO₂ nanoparticles. Results indicated that the CeO₂ nanoparticles had uniform particle size of 2 nm, and AA andEHA/AA were successfully modified on the surface of CeO₂ nanoparticles. The as-prepared EHA-capped and EHA/AA-capped CeO₂ nanoparticles could be uniformly dispersed in the AN5 base oil and maintained for 90 days without settling. Tribological performance test showed the as-prepared EHA-capped and EHA/AA-capped CeO₂ nanoparticles exhibited good film-forming ability during sliding and could effectively improve the friction-reducing and antiwear abilities as well as extreme pressure properties of the base stock. The results of QCM-D showed that EHA/AA-capped CeO₂ nanoparticles could be adsorbed on metal surface faster and with larger adsorption thickness. In addition, EHA/AA-capped CeO₂ nanoparticles had better anti-wear properties, due to the carboxyl active groups contained in the surface modifier, EHA/AA-capped CeO₂ nanoparticles were more easily adsorbed and deposited on the metal surface, so that they could be adsorbed and deposited on the friction surface more quickly, forming a more denser and thicker adsorption layer to protect the friction surface. As the friction progresses, cerium oxide nanoparticles participated in the tribochemical reaction to form a nanocomposite lubrication film.