Abstract: A new mixed phosphate was prepared and characterized by Fourier transform infrared spectroscopy (FT-IR). Tribological properties of the new phosphate as an additive of ester oil for a steel/steel contact were investigated on an optimal SRV friction and wear tester. The morphology and element analysis of the worn scar surfaces were characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The experimental results showed that the novel additive showed excellent anti-wear and load-carrying capacity. XPS results showed that a boundary lubricating film was formed on the steel worn surface by physical and chemical adsorption, while tribochemical reaction of the additive with the steel surface was involved at elevated temperature to generate a boundary lubricating film containing fluoride, chloride and phosphate. This contributed to increased load-carrying capacity and reduced wear.