In this work, the effects of atmospheric-pressure plasma treatment of polyimide films on surface and dielectric characteristics were studied by using X-ray photoelectron spectroscopy (XPS), surface free energies, and dielectric spectrometer. The total surface free energies and dielectric constants of plasma treated polyimide film were deareased with increasing the treatment time, which was mainly due to a lower London-dispersive component, while the specific component was increased. This could be attributed to an increase of the polar fluorine groups of polyimide per unit surface area. And the fluorine content of the polyimide film was increased with increasing the amount of the treatment time, resulting in decreasing dielectric constant of the film. It was found that the replacement of fluorine led to the decrease of the local electronic polarizability of polyimide, or to the increase of the free volume, which could be attributed to the relatively large volume of fluorine.