Thermal stabilities of indium tin oxide (ITO) substrates and TiO2/ITO structures were evaluated in relation to their electrical properties. The ITO substrates and TiO2/ITO structures were annealed at 350, 400, and 500 °C. The ITO substrate with large grain size showed higher thermal stability than that with small grain size. The thermal stability of TiO2/ITO structure improved with increasing TiO2 thickness, and a decrease in electron concentration was observed in resistance-increased samples. These changes were attributed to variations in grain-boundary potential caused by oxygen adsorption. It may be concluded that variation of the grain-boundary potential by thermal annealing has a dominant influence on resistance. Therefore, optimization of the grain size is important to improve the thermal stability of ITO. This mechanism and procedure can be applied to improve the characteristics of other TCO materials.