Abstract: Based on the developed oil film thickness measuring system for high-speed ball-on-ring contact, the EHL test was conducted with base oil PAO6 at high speed, where the steel ball was driven by the ring in nominally tractive rolling mode for simulating the contact between ball and outer ring in ball bearings. The experimental results show that the measured central film thickness under high speed greatly deviated from the classical EHL prediction. The thermoelastohydrodynamic lubrication model was utilized to explore the factors that resulted in the reduction of film thickness at high speeds. The calculation results show that the ball and ring were not in pure rolling mode, but in severe rolling with sliding state, which was verified by measuring the speed of steel ball. And then combined with the calculated results of temperature in the contact zone, the mechanism of elastohydrodynamic lubrication behavior at high speeds was concluded that the high slide-roll ratio resulted in the entrainment speed less than the theoretical value, which was the key reason for the decrease of film thickness at high speed; the thermal effect resulting from the high slide-roll ratio further induced the decrease of film thickness.