Di-n-hexadecyldithiophosphate (DDP)-coated ZnS nanoparticles and non-coated ZnS nanoparticles were chemically synthesized and characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and thermogravimetric analyzer (TGA). The antiwear ability of the DDP-coated ZnS nanoparticles and non-coated ZnS nanoparticles as additives in liquid paraffin were evaluated on a four-ball machine, the morphologies of the worn surfaces were examined by means of scanning electron microscopy (SEM). The results show that DDP-coated ZnS nanoparticles in an average diameter of about 3 nm are able to prevent water adsorption and to disperse stably in organic solvent or mineral oil. DDP coating on the surface of ZnS nanoparticles shows higher thermal stability than PyDDP, DDP-coated ZnS nanoparticles as additive in liquid paraffin show better antiwear ability than non-coated ZnS nanoparticles, owing to the formation of a boundary film.