Copper oxide (CuO x ) thin films were deposited on flexible polyethyleneterephthalate (PET) substrates by rf magnetron sputtering and the effects of the rf power and the O 2 fraction on the physical and chemical properties of the films were examined. The obtained films were characterized with a surface profiler, X-ray diffraction (XRD), scanning electron microscopy (SEM), four points probe, and X-ray photoelectron spectroscopy (XPS). The trend of surface resistance of CuO x films was almost identical as rf power varied from 200 to 300W. The surface resistance of the films increased as the O 2 fraction increased up to 14.3%. After this point it decreased. The propensity of surface resistance of the films, however, monotonically increased as a function of O 2 fraction at 400W of rf power. XRD study revealed that the preferential phase of CuO x changed from metallic cubic Cu(111) through cubic Cu 2 O(111) to monoclinic CuO(1¯ 1 1) as the O 2 fraction increased. The compositional ratio of Cu species with different oxidation state in CuO x films was investigated by X-ray induced Auger electron spectroscopy (XAES). XAES study revealed that the compositional ratio of Cu was dependent with the rf power. The higher rf power applied, the more Cu + species and less Cu 2+ species formed in CuO x films.