The determination of hydrodynamic film failure has become one of the key aspects in the study of thin film lubrication (TFL) since the hydrodynamic effect of fluid film at nano-scale can be observed with recently developed experimental techniques. In the present paper, the relative optical interference intensity (ROII) technique with a resolution of 0.5 run in the vertical direction has been used to measure the film thickness. Experimental results show that the hydrodynamic effect can be clearly observed even at very low speed if the contact pressure is sufficiently low or if the viscosity of lubricant is comparatively high. When the pressure increases to a certain degree, the film will suddenly drop to the dimension of several layers of molecules and this is where the failure of the fluid film has taken place. For different viscosity of lubricants, the fluid film failure occurs at different rolling speeds and pressures. In addition, when the normal load becomes higher, a higher speed or larger viscosity is required to form the fluid film in the contact region. Finally, the effects of pressure, viscosity, and velocity on the occurrence of fluid film failure have been examined and a relationship involving the three parameters is proposed.Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998