This study investigated the effect of copper (Cu) nanoparticles on the solid contact between friction surfaces by applying a molecular dynamics method to reveal the mechanisms responsible for the favorable friction properties of nanoparticles. Two models were built, which were named model A (without Cu) and model B ( with Cu), respectively. The differences in the mechanical properties between these two models were compared. The simulation results demonstrated that the improvement in friction properties by Cu nanoparticles was more obvious at low velocity than at high velocity. At low velocity, a Cu nano-film was formed on the friction surface, which accommodated the velocity gradient and plastic deformation. Due to the good lubrication effect of the nano-film, the plastic deformation, defect structures and friction force of model B were improved compared with model A. Under high velocity conditions, a transfer layer appeared adjacent to the interface in both models. Because of this, the friction forces of the two models decreased with increased velocity. The fluid mechanics theory was used to explain why the friction force in model B was lower than that in model A at high velocity. The effect of the load on friction properties was also analyzed and the results showed that the mechanisms of anti-wear and friction reduction by Cu nanoparticles under a low load were the same as those under a high load. (C) 2014 Elsevier B.V. All rights reserved.