Thermal barrier coatings are applied to aeronautical and industrial gas turbine components to protect from detrimental effects of hot gases. Thermal barrier coating systems are generally composed of a substrate material, an oxidation resistant metallic bond coat, and a thermal protective ceramic-based topcoat. Additionally, thermally grown oxide layer is formed at ceramic/bond coat interface as a result of exposure of bond coat to high temperature. Oxidation mechanism is one of the major failure mechanisms of thermal barrier coatings. Thermally grown oxide layer plays significant role as a oxygen barrier, but rapid thickening of thermally grown oxide leads to spallation failure of thermal barrier coatings. In this study, thermally grown oxide growth behavior was investigated at isothermal oxidation condition to evaluate durability of the thermal barrier coating system. The thermal barrier coating system consists of yttria stabilized zirconia (YSZ) topcoat and NiCrAlY bond coat deposited on Inconel 718 superalloy with atmospheric plasma spray technique. After coating process, specimens were exposed to 900 degrees C air atmosphere for different periods up to 50 h. Ceramic/bond coat interface and thermally grown oxide layer were examined using scanning electron microscopy analysis. Besides, porosity contents and microhaidness measurements were carried out to determine strength of coating. The results showed that thickness of thermally grown oxide layer increased and porosity rates of ceramic layer decreased with the effect of oxidation. Accordingly, the ceramic layer hardness increased due to high temperature effect.