The aim of this paper is to investigate the effect of ultra-violet (UV) ray irradiation on the crack formation of brittle ceramic coating on polymer substrate. It is well known that ultra-violet ray irradiation degrades the mechanical properties of polymer and polymer-based coating films. We carried out the tensile tests of PET/ITO film specimen after UV irradiation under the microscope and observed the crack formation on ITO surface. Also, we carried out nano-indentation tests of PET substrate after removing ITO layer to characterize the change of the mechanical properties near the interface between PET and ITO. The results show that the number of cracks vertical to loading direction and the rate of vertical crack formation decreased after UV irradiation. Hardness and Young’s modulus of PET substrate increase due to the oxidation of PET after UV irradiation. To explain the relationship between the crack formation and mechanical property change, the energy release rates of the thin film channeling cracks are considered.