The aim of this paper was to investigate the tribological properties of magnetite (Fe 3 O 4 ) nanoparticles (NPs) with various morphologies (i.e., hexagonal, octahedral, and irregular morphologies) utilized as lubricating additives on a four-ball tribo-tester. The diameters of wear scar and the friction coefficients were measured using a scanning electron microscope (SEM) and a statistical software, respectively. The properties of the friction pair surfaces were examined by a SEM, atomic force microscope, energy dispersive X-ray spectroscope, and X-ray photoelectron spectroscope. The results show that the Fe 3 O 4 NPs, especially the NPs with hexagonal morphology, have superior antifriction properties as lubricating additives. In addition, the antifriction mechanism of the Fe 3 O 4 NPs as lubricating additives was also discussed.