Abstract: A surface free energy difference criterion,i.e.the surface free energy difference between condensate liquid at condensation temperature and solid surface, was proposed to predict whether filmwise or dropwise condensation of a vapor occurs on a solid surface.The critical surface free energy difference was considered to be 0.0333J·m ^-2 .The surface free energy of a solid can be calculated from the measured contact angles at room temperature for low surface-free-energy solid surfaces like polymer films.Consequently,it is more convenient and accurate to use the surface free energy rather than the contact angle to predict condensation mode because the surface free energy difference is independent of the temperature for contact angles measurement.It is of considerable significance to get more insights into the mechanism of condensation heat transfer enhancement by means of surface coatings.Meanwhile, the criterion is much helpful to choosing the coating materials that promote dropwise or dropwise-film coexisting condensation modes according to the condensing vapor.