We prepared Na0.7CoO2 thin films on (001) Al2O3 substrates that showed a negligibly strained structure and a metallic behavior similar to Na0.7CoO2. single crystal. In particular, a Na0.7CoO2 thin film on a (111) SrTiO3/(001) Al2O3 Substrate exhibited an elongated c-lattice constant as well as an anomaly in the electric transporting behavior: a transition from an insulating state to a metallic state at 105 K. This change is Caused by the structural symmetry break near the cubic-to-tetragonal transition temperature of SrTiO3. From spectroscopic ellipsometry analysis, we obtained optical conductivities for the Na0.7CoO2 thin films on the Al2O3 and SrTiO3/Al2O3 substrates. The Na0.7CoO2 thin film on the SrTiO3/Al2O3 substrate exhibited higher energy splitting between e(g) and a(1g) than the Na0.7CoO2 thin film on the Al2O3 substrate, resulting from the c-lattice elongation. (C) 2008 Elsevier B.V. All rights reserved.