Indium tin oxide/polyethylene terephthalate (ITO/PET) thin film/flexible substrates are widely applied in dye-sensitized solar cells, touch screens, and energy efficient windows, In this study, an application of the digital image correlation (DIC) method was used to determine the coefficient of thermal expansion (CTE) of ITO/PET thin films, and the effects of thickness variations of ITO and PET on the CTE of the samples. The observation range of the experimental temperature was selected from room temperature to the glass transition temperature of PET, which is 70°C. A novel DIC experimental process reduces errors caused by variations in the refractive index of the surrounding heated air is proposed. As a result, the error of the CTE measurement was reduced from 15 to 23% to less than 4%. The experimental results demonstrated that the CTE of the ITO/PET sample was anisotropic. Furthermore, the CTE of an ITO/PET sample increased by decreasing the thickness of PET flexible substrates, and increased by increasing the thickness of ITO film, and thus decrease the surface resistance of ITO film.