A new kind of oil additive, stearic-acid-coated (NH4)3PMo12O40 nanoparticles was synthesized by a chemical method. The morphology and structure was characterized by means of transmission electron microscopy; X-ray powder diffraction and Fourier transform infrared spectroscopy. The tribological properties of coated (NH4)3PMo12O40 nanoparticles as oil additives in liquid paraffin were evaluated in a four-ball machine. The results showed that the coated (NH4)3PMo12O40 nanoparticles have an average diameter of about 20 nm, and the (NH4)3PMo12O40 nanocore has a Keggin structure. As oil additives they dramatically improved the antiwear ability of liquid paraffin, and enhanced the friction-reduction and load-carrying ability of liquid paraffin to some extent. The rubbed surface was investigated by scanning electron microscopy and X-ray photoelectron spectrometry. It was found that the boundary film on the worn surface was composed of MoO3, (NH4)2MoO7 and Fe2O3. Which contributed to the good tribological properties of the oil.