In order to describe the mixed lubrication in nano-scale which is constituted from dry friction, boundary lubrication, and thin film lubrication, a contact ratio between surfaces of a glass disk and a steel ball in a pure rolling process has been measured by the technique of Relative Optical Interference Intensity (ROII) with a resolution of 0.5 nm in the vertical direction and 1 μm in the horizontal direction. The relationships between the contact ratio and its influence factors have been investigated. Experimental results indicate that the contact ratio in the static state is related to the combined surface roughness, maximum Hertz pressure, and the combined elastic module of tribo-pair in an exponential function. The decrease of rolling speed or lubricant viscosity, and the increase of the pressure will enhance the dynamic contact ratio which is the contact ratio measured in the rolling process. The addition of polar additives into basic oil will reduce the contact ratio. The contact ratio between rough surfaces is larger than that between smooth surfaces in the higher speed region. However, the former becomes smaller than the later after speed decreases below a critical value. A formula for calculating the dynamic contact ratio is given in the end of the paper.