Poly (ethylene terephthalate) (PET) and nylon 6 films were treated with low temperature plasma in various gases, oxygen, nitrogen, hydrogen, helium, argon, methane, tetrafluoromethane, trifluoromethane, monochlorotrifluoromethane, and tetramethylsilane. The contact angles of various liquids of different surface tensions on the plasma-treated films were measured. The lowest surface tension γc (Zisman) and the highest surface tension γc (max) of the polymers were determined on the basis of the Zisman plots. Furthermore, the contributions of nonpolar dispersion, dipole, and hydrogen bonding forces to the surface tensions for the plasma-modified polymers were evaluated by the extended Fowkes equation. In addition, the plasma-treated samples were characterized by X-ray photoelectron spectroscopy (ESCA). The change in surface free energy of the plasma-treated films was correlated with their surface chemical compositions measured by ESCA.