The paper investigates their tribological performance and lubrication mechanism of highgrade commercial engine oils (Shell,Castrol and MGSS) SN 5W40.Fourball and reciprocal tribometers were used to investigate tribological performance of engine oils respectively.The contained elements in the lubricating oils were analyzed by XRay fluorescence (XRF).Solid particles obtained from the oils by centrifugal process were characterized by transmission electron microscope (TEM).Experimental results indicated that three kinds of commercial lubricating oils contained nanoparticles with different structure.Nanoparticles might be easy to penetrate into friction contacting area and the lubricities of lubricating oils were improved.