The surface of polyimide films was fluorinated and a CF3 surface layer formed. The dielectric constant of the polyimide films decreased to a minimum of 2.5 as the treatment time increased. The leakage current of the polyimide films was also decreased significantly by the fluorination, The measurement of electrical properties and secondary ion mass spectroscopy revealed that the fluorine atoms diffused into the polyimide films from the fluorinated surface layer. The current-voltage characteristics indicated that reduction in the leakage current was due to fewer ionic impurities. This confirms that the fluorine diffused into the polyimide and it eliminated impurity ions.