The oxidation of Zn films was attempted by irradiation with an excited metastable atom beam of noble gas (He, Ne, Ar or Kr) in an oxygen atmosphere of the order of 10(-3) Pa. The excited metastable atoms were generated in a glass tube by an RF discharge and were ejected through a nozzle to a Zn film surface, to which a negative bias voltage was applied. The presence of the excited metastable atoms on the surface was confirmed by detection of the electron current ejected from a Cu plate which was irradiated with the excited metastable atom beam. For Ar and Kr, Zn films were hardly oxidized. On the other hand, transparent ZnO films were obtained for He and Ne. Judging from the fact that the energies of the excited metastable states of He and Ne are higher than the ionization potential of O-2 molecules, the oxidation of Zn films is attributed to the generation of O-2(+) molecular ions on the film surface through the Penning ionization process by the excited metastable atom beam.