Abstract: The nanoparticles of molybdenum disulfide were prepared by heating molybdenum trisulfide precipitates obtained from the aqueous reaction of sodium sulfide and sodium molybdenate in hydrogen atmosphere. The crystallite composition and micro-morphology of the resulting MoS_2 nanoparticles were analyzed by means of X-ray diffraction and transmittance electron microscopy. The tribological behavior of the MoS_2 nanoparticles as additive in N46 machine oil was comparatively investigated with that of commercial micro-MoS_2 particulates on a four-ball machine. The chemical states of the elements on the worn steel surface under different lubricating conditions were determined on an X-ray photoelectron spectroscope, while the morphologies of the worn steel surfaces lubricated with different lubricant systems were observed on a scanning electron microscope. It was found that the MoS_2 nanoparticles had a grit size of 20-30 nm. They showed better friction-reducing, antiwear, and extreme pressure properties than the commercial micro-MoS_2 in the N46 machine oil. This could be attributed to the easier adsorption of the MoS_2 nanoparticles on the sliding steel surface and the formation of a molybdenum trioxide-containing lubricating and protective film thereon by the readily oxidation of the nanoparticles in the sliding process.