Poly(ethylene terephthalate) (PET) films were treated with CF4 plasma immersion. The samples were processed at different RF powers and treatment time. The surface modification of PET films was evaluated by water contact, angle (CA), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). Decrease in contact angle of both sides of PET films was observed under mild treatment conditions. However, as raising treatment power and/or time, the change in contact angle between the two sides of PET films was different. The relatively hydrophobic and hydrophilic surfaces were being in situ formed on the two sides of PET films, respectively. And the extreme values of water contact angle reached 108.63 and 7.56 degrees, respectively. XPS analyses revealed that there was a substantial incorporation of fluorine and/or oxygen atoms in both side surfaces. The relative chemical composition of the C (is) spectra's showed the incorporation of non-polar fluorine-based functionalities (i.e. -CF-CFn-, -CF2- or -CF3 groups) and polar oxygen-based functionalities (i.e. -COOH or-OH groups) in the surfaces. Correlation between the plasma parameters and the surface modification of PET films is also discussed. (c) 2005 Elsevier B.V. All rights reserved.