It is found that the critical transition film thicknesses from elasohydrodynamic lubrication (EHL) to the thin film lubrication (TFL) and from the thin film lubrication to the boundary lubrication (BL) are related to the initial viscosity of lubricant, surface energy of substrates, etc.. In order to complete lubrication theory system, a lubrication model is proposed in this paper. In the model, three basic films, i.e. the dynamic film, the ordered lubricant film and static film, are proposed to represent the hydrodynamic lubrication, thin film lubrication and boundary lubrication. According to the division, we can tell different regimes from the friction of each basic film in the whole film. Since these basic films have their thickness region, the transitions between EHL, TFL and BL can be determined when the molecular structure of a lubricant is known.