Two phase lubricant has attracted attention as a lubricating oil with little change in viscosity with temperature change. The two phase lubricant consist of the low and high viscosity base oils which are miscible at high temperature but not at lower temperature. For exmaple, Mineral or synthetic base oil are used as a low viscosity lubricant and PAG（polyalkylene glycol）. In the high temperature region, PAG dissolves into mineral or synthetic base oil and the viscosity reduction is relieved. However, the structure and frictional characteristics of the oil film at the sliding part have not been elucidated. In this paper, in-situ analysis of the oil film formed on the friction surface was carried out with a reflection spectroscopic film thickness meter. And, this paper clarified the influence of the oil film structure and oil film thickness of the two phase lubricant and its friction characteristics. As a result, it was suggested that the oil film of the two phase lubricant in the sliding part was a multilayer due to the adsorption layer of PAG. In addition the two phase lubricant showed a friction coefficient lower by about 0.03 to 0.07 at the experimental temperature compared to PAO, and it was clarified that a thick oil film was formed.