The addition of nanoparticles improves the anti-wear and friction reduction behaviors. The roles of nanoparticles in sliding friction process were investigated at the nanoscale using molecular dynamics simulations. The agglomerated nanoparticles deposited onto the friction surface and formed the protecting film to reduce the friction. Both the rolling friction and sliding friction existed in the friction process. Importantly, the nanoparticles were inclined to fill into the pit which smoothed the friction surface, but a hole in the pit was still observed which could decrease the surface quality.