Abstract: The purpose of this paper is to obtain more understanding about the how lubricants perform under practical conditions such as high contact pressures and elevated temperatures. A test rig is designed and manufactured to obtain rather reliable results of film thickness of gear oils confined between narrow gaps at extreme situations. Relative optical interference intensity was used to research film-forming properties of five gear oils in rolling, concentrated contacts under different pressures and temperatures. The main effect of contact pressure on film-forming properties of gear oil was that film thickness obviously decreased when more pressure was applied. Temperature, however, can influence both of film thickness and lubricating regime. In this study GL-5 85W/190 and GL-5 85W/90 can form rather thick film at high pressure up to 3 GPa and a temperature of 120 ℃ while the other three gear oils shared similar weak performance at extreme conditions.