With a view to the fact that lubricant molecules usually have chain structure, a physical model has been established to analyze the lubrication characteristics of nano thin film, by correlating the molecular chain length of the lubricants and the film thickness. Thus the equivalent Reynolds equations for the nano thin film lubrication have been derived from the basic equations of fluid mechanics, by taking into account the rotation terms. Numerical analysis has also been carried out to investigate the influence of the characteristic length of the lubricant molecules on thin film lubrication performance. Results indicate that the load carrying capacity of the thin lubricating film calculated by the present model is significantly larger than that based on the conventional fluid lubrication theory for a thicker film. This is because the length effect of the lubricant molecules is significant in thin film lubrication. Moreover, the range of thin film lubrication and the relationship between the film thickness and velocity determined with the present model agree well with the reported experimental ones.