The authors have fabricated Nax Co O2 thin films via lateral diffusion of sodium into Co3 O4 (111) epitaxial films (reactive solid-phase epitaxy [Ohta, Cryst. Growth Des. 5, 25 (2005)]). The environment of thermal diffusion is key to the control of the sodium content in thin films. From the results of x-ray diffraction and in-plane ρ ab, the epitaxial growth and the sodium contents of these films were identified. The thermoelectric measurements show a large thermoelectric power similar to that observed in single crystals. The quasiparticle scattering rate is found to approach zero at low temperatures, consistent with the small residual resistivity, indicating high quality of the Nax Co O2 thin films.