The di-n-hexadecyldithiophosphate (DDP) surface-modified ZnS nanoparticles were chemically prepared. The particle size of the DDP-coated ZnS nMoparticles was characterized by transmission electron microscopy (TEM). The antiwear ability of the coated ZnS nanoparticles as an additive in liquid paraffin was investigated using a four-ball machine. TEM image confirms that the average diameter of the DDP-coated ZnS nanoparticles is about 4 nm. Wear tests show that DDP-coated ZnS nanoparticles as additive in liquid paraffin is capable to reduce wear of steel even at an extremely low concentration. According to the results of X-ray photoelectron spectrometer (XPS) to the boundary film, a physically adsorbing and tribochemically reacted film with ZnS, ZnO, FeS, Fe2O3, SO42- and PO43- were formed during friction process. which is contributed to goad tribological properties of DDP-coated ZnS nanoparticles. (C) 2000 Elsevier Science S.A. All rights reserved.