Oleylamine (OM) and maleic anhydride dodecyl ester (MADE, synthesized at our laboratory) were adopted as the surface modifiers to prepare OM/MADE-capped tungsten disulfide (WS2) nanoparticles. An X-ray diffractometer and a transmission electron microscope were performed to analyze the microstructure and phase ingredients of the OM/MADE-capped WS2 nanoparticles. Moreover, a four-ball friction and wear tester and a reciprocating tribometer were employed to evaluate the tribological properties of the surface-capped WS2 nanoparticles as the lubricant additive in diisooctylsebacate (DIOS) from room temperature to 150 degrees C. The morphology of the worn steel surfaces and wear scars and their chemical states were investigated with a scanning electron microscope, three-dimensional profilometry, and an X-ray photoelectron spectroscope. Results show that OM-capped WS2 nanoparticles nearly have no effect on the tribological properties of the DIOS base oil. The OM/MADE-capped WS, nanoparticles added in the same base stock at a concentration of 2.0% (mass fraction), however, exhibit good dispersibility and result in greatly improved tribological properties. The reason lies in that, after surface-capping by MADE containing polar group and OM containing coordination group, the OM/MADE-capped WS, particulates added in the base oil are well adsorbed on the sliding surfaces of the steel steel contact to afford a chemisorption film with a low shear force. At the same time, OM/MADE-capped WS, nanoparticles as the additive in DIOS base oil take part in tribochemical reactions to form tribofilm composed of WO3 and iron oxides on sliding surfaces, which also contributes to reducing the friction and wear of the steel sliding contact.