Abstract: A four ball machine was used to investigate the tribological behavior of oleic acid (OA) coated PbO nanoparticles as additive in liquid paraffin. A comparative investigation was also carried out on the same tester with respect to lead oleate as an additive in liquid paraffin. The worn surfaces of the steel balls were analyzed by means of X ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). As the results, OA coated PbO nanoparticles as additive effectively increases the friction reduction and antiwear ability of liquid paraffin. It shows better friction reduction and antiwear performance than lead oleate as well. The analytical results of XPS, SEM and EDS indicate that a boundary film mainly composed of PbO is formed on the steel ball surface under the lubrication of liquid paraffin containing OA coated PbO nanoparticles. This contributes to improve the friction reduction and antiwear behavior of the base stock.