We studied the influence of the mechanical strength of dielectric thin films on the durability against the thermal stress cycle during repetitious overwriting in phase-change optical recording. Dielectric thin films such as tantalum oxide (TaOx), silicon nitride (SiN2), and a compound of zinc sulfide and silicon dioxide (ZnS:SiO2), were applied for the protective layers that sandwich a GeSbTe alloy active layer. Thermal stress that led this sandwiched medium to fail must mainly be associated with a deformation due to melting or amorphization of the active layer, and a thermal expansion of the solid dielectric protective layers. The thermal stresses during repetitious overwriting caused microstructural changes in the protective layers : a plastic deformation, and cracking and void formations. The stresses also caused a deformation of a substrate surface below the bottom protective layer. These microscopic defects accumulated, and subsequently resulted in optically detectable macroscopic defects. TaOx and SiNx films having high density, especially of more than 80% of the bulk density, could suppress the plastic deformation during overwriting effectively, whereas nucleated cracking defects were easily enlarged due to their brittleness. Contrary to these brittle films, ZnS:SiO2 could suppress the cracking propagation by its plastic deformation.