The tribological behavior of copper nanoparticles suspended in oil is investigated experimentally. The friction coefficient for raw oil and nano-oil mixed with copper nanoparticles is investigated by using a disc-on-disc tribotester between mixed and full-film lubrication regime. The friction surfaces are investigated by using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and atomic force microscopy (AFM). The results show that the average friction coefficient for nano-oil with 25 nm and 60 nm copper nanoparticles under a load of 3000 N is decreased by 44% and 39%, respectively. It is found that the nano-oil mixed with copper nanoparticles has a lower friction coefficient and less wear on the friction surface, indicating that copper nanoparticles improve the lubrication properties of raw oil. (C) 2009 Elsevier B.V. All rights reserved.