Abstract: Nanometer copper borate with a particle size of about 10~20 nm was prepared using carbon dioxide supercritical dry technique. The tribological properties of 500 SN oil containing nanometer copper borate were investigated with a four ball machine and a block on ring friction and wear tester. The morphology and chemical features of the wear scar were analyzed by means of SEM and XPS. As the results, the load carrying capacity and antiwear ability of the base stock were increased by the additive. The friction coefficient, however, was slightly increased by addition of the additive in the base stock. XPS analysis of the wear scar indicates that B 2O 3 and FeB were formed on the rubbing surface, which provides the oil with higher load carrying capacity and better antiwear ability.