ZnS nanoparticles coated with di-n-hexadecyldithiophosphate (DDP) were chemically synthesized. The structure of the prepared ZnS nanoparticles was investigated by means of transmission electron microscopy, electron diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The thermal stability of DDP coated on nanoparticles was compared with that of pyridinium di-n-hexadecyldithiophosphate (PyDDP) using a thermogravimetric analyzer. The tribological properties of ZnS nanoparticles as an additive in tetradecane were investigated by a SRV tester in a ball-on-disk configuration. DDP-coated ZnS nanoparticles, with an average diameter of about 4 nm, are able to prevent water adsorption, and oxidation and are capable of being dispersed stably in organic solvents or mineral oil. Thermal stability of DDP coating on ZnS nanoparticles was superior to that of PyDDP. Wear tests show that DDP-coated ZnS nanoparticles as additive in tetradecane are capable of reducing friction and wear of steel.