The mechanism of the transition from elastohydrodynamic lubrication (EHL) to thin film lubrication (TFL) has been studied with a technique of relative optical interference intensity for measuring the film thickness. The resolution of the instrument could reach 0.5 μm in the vertical direction and 1.5 μm in the horizontal direction. The factors influencing the measurement precision and the differences between EHL and TFL are discussed. Film thicknesses with different lubricants, velocities, and loads are measured. The experimental data show that the variation of the film thickness in TFL is not the same as that in EHL. As the rolling speed decreases, a turning point in the film thickness curve is found, which distinguishes TFL from EHL. Consequently, a TFL model with a fluid layer, an ordered liquid layer, and an adsorbed layer is proposed. Finally, the functions of the layers are discussed in detail.