We studied the effects of $O_2$ plasma on the hydrophilic/hydrophobic contrast of film substrates. PET (polyethylene terephthalate) and FEP (fluorinated ethylene propylene) films were treated by $O_2$ plasma, and the surface of the films was investigated using atomic force microscopy(AFM), X-ray photoelectron spectroscopy (XPS), and contact-angle and surface energy measurements. Surface energies were calculated from the measured contact angles between several solutions and the films based on the geometric mean and Lewis acid-base method. The contact angle of PET films treated by $O_2$ plasma decreased with increasing treatment time in water and surfactant solution, and surface energy increased with increasing polar component of surface tension of the film. In contrast, the contact angle of the $O_2$ plasma-treated FEP film decreased, and surface energy decreased with decreasing nonpolar component of surface tension of the film.