Nanofluids containing nanocomposite materials have broad application prospect in lubrication. In the present study, MoS2-Al2O3 nanocomposite was synthesized by solvothermal method. Then water-based nanofluid containing MoS2-Al2O3 nanoparticles was prepared and it exhibited optimal stability and wettability. The tribological properties of MoS2-Al2O3 nanofluid under steel/steel contact were investigated. Through chemical analysis of worn surface, tribochemical reactions occurred and tribofilm generated at friction interface were studied to reveal the lubrication mechanism. The results showed that MoS2-Al2O3 nanofluid exhibited outstanding effect on reducing the friction force and wear rate of friction pairs. The superior lubrication performance was partly attributed to the entry and movement of nanoparticles at friction interface. More importantly, due to interfacial tribochemistry, a tribofilm with double layer structure composed of adsorption film and reaction layer was formed on the metal surface, preventing the direct contact of steel surfaces. The adsorption film with a thickness of about 16 nm was predominated by amorphous substrates and crystal phases (ultrafine Al2O3 and MoS2 debris) embedded in the top. And the reaction layer beneath adsorption film contained high mechanical properties iron oxides (mainly Fe3O4 and Fe2O3) and self-lubricating Fe2(SO4)3, which further alleviated friction and enhanced the wear-resistance of materials. Display omitted •MoS2-Al2O3 nanocomposite was synthesized and applied as lubricating additive.•MoS2-Al2O3 nanofluid showed excellent performance under steel/steel contact.•A double-layered tribofilm due to interfacial tribochemistry was formed on surface.•Iron oxides and Fe2(SO4)3 in reaction layer further alleviated friction and wear.