A dimer acid diethanolamine salt was synthetised by dimer fatty acid and diethanolamine.The tribological properties of the salt in water-based fluid were evaluated with a four-ball machine.The typical elements on the worn surfaces of steel ball were analyzed by X-ray photoelectron spectroscopy (XPS).The results indicate that the synthetised additive possesses good load-carrying properties,antiwear and friction reducing abilities as well as good rust-inhibiting and anti-corrosion performances in water-based fluid.An adsorption film containing nitrogen and a chemical reaction film containing chemical species of Fe2O3 are formed on the steel ball surfaces lubricated with dimer acid additive during the friction process.The cooperation action of these films contributes to the improvement of tribological properties of the lubricant.