When Thermal Barrier Coating (TBC) is subjected to a high-temperature environment, thermally grown oxidation (TGO) layer is grown at interface between top coating and bond coating. The existence of oxidation will bring reduction of coating cohesion. The aim of this study is to clarify TGO growth process under variation of temperature condition. For this purpose, one-stage variation of temperature tests, which is one that after heating TBC specimen at constant temperature 973 K during a constant period 500 h, specimen is heated additionally at 1 173 K, for instance, was conducted. TGO growth process was examined by SEM observation of a cross section of the aged specimen. As some results obtained, (1) TGO growth process had no effect of difference in a kind of substrate material. (2) TGO layer was not formed at aged temperature below 973 K. In aging temperature range between 1 073 K and 1 173 K, Al2O3 as TGO layer was formed at interface between top coating and bond coating layer.(3) TGO grew under post-aged temperature, which is higher than pre-aged temperature. And then, TGO growth law at a constant temperature is also effective to complicated condition as a variation of temperature focused in this study.