Epitaxial Na0.5CoO2 thin films were fabricated by the sodium deintercalation of the epitaxial Na0.7CoO2 thin films in the solution of iodine-dissolved acetonitrile, in which the epitaxial Na0.7CoO2 thin films were deposited on (001) sapphire substrates by pulsed laser deposition. The Na0.7CoO2 thin films show small resistivity, which is probably due to the high crystallinity as well as the compressive strain of 1.1% along the a-axis. From the spectroscopic ellipsometry analysis, we obtained the optical constants as well as the optical conductivities for the Na0.5CoO2 and Na0.7CoO2 thin films and the increased energy splitting between eg and a1g results from the structural strain of the Na0.7CoO2 thin film. We suggest that the structural strain should be the source of the lower resistivity and the preservation of the strongly correlated system up to 200 K for the Na0.7CoO2 thin film. At 300 K, the Na0.7CoO2 thin film exhibits the enhanced power factor (S2/ρ) of 94μW/K2 cm with the low electrical resistivity of 177μΩ cm and the high thermoelectric power of 1- 28.9μV/K.