NaxCoO2 thin films were fabricated by means of RF-magnetron sputtering. We measured and analyzed the thermal properties and changes of the NaxCoO(2) crystal structure by XRD, SEM, Raman spectra, and XPS analyses. Sodium ions diffused from the bulk of the thin film to the surface as the temperature increased. The diffused Na ions reacted with oxygen ions and Na2O was formed on the surface of the thin film, resulting in a decrease of the carrier concentration and a change of the crystal structure from a layer to a spinel structure. The Seebeck coefficient of the NaxCoO(2) thin film annealed at 550 degrees C is larger than the value (100 mu V/K) for single crystal NaCo2O4.