Thermal barrier coating (TBC) of a gas turbine blade suffers from high temperature oxidation. It is known that thermally growth oxidation (TGO) grows at the interface between ZrO2- 8%Y2O3 and CoNiCrAlY, and the TGO degrades the adhesive strength. The purpose of this study is to clarify the TGO growth process. Thermal aging tests of ZrO2-8%Y2O3 / CoNiCrAlY TBC systems under various temperature conditions were carried out. TGO growth process was observed by an electron probe micro analyzer (EPMA). Both TGO of Al and TGO of other elements were examined, and the thickness of the TGOs were examined. Results are summarized as follows. (1) The delamination occurred at a ZrO2-8%Y2O3 layer beside the interface. (2) The growth rate of complicated TGO which consisted of Co, Ni and Cr increased with an increase of temperature. However, the complicated TGO wasn’t related with the delamination life. (3) The delamination could occur if the thickness of Al TGO increased more than about 3 μm.