Thermal barrier coating (TBC) is used to protect hot parts of gas turbine from a high-temperature environment. Recently, TBC delamination damage was also reported in some Japanese electric power plants. It is important to establish a TBC delamination evaluation-method for preventing such damage. It is known that the delamination is due to a reduction of interface cohesion between coating layer and substrate, and then the reduction is caused by a formation of thermally growth oxidation (TGO) layer at the interface. Thus, it is necessary to understand TGO layer growth behaviour and the growth mechanism. The aim of this study is to examine effect of an aging temperature and an aging time on TGO layer growth under various aging temperature conditions, and then to discuss a TGO growth mechanism through SEM observation of a cross section of aged TBC specimen. As some results obtained, TG0 layer was not formed at the aged temperature below 973 K. In aging temperature range between 1 073 K and 1 173 K, Al2O3 as TGO layer was formed at the interface between top coating and bond coating layer. At aging temperature 1 273 K, (Co, Cr, Al) complex oxidation in addition to Al2O3 at the interface was observed. It was shown from TGO growth curve that the growth exponent n was 0.3 in aging temperature range 1 073 K to 1 173 K, and then 0.1 at 1 273 K in contrast with 0.5 for a metal oxidation process. Then, TGO layer grew by diffusion of oxidizing species on grain boundary in coating layer. Additionally, the reason that the growth exponent at 1 273 K was reduced was due to what the complex oxidation layer was acted as barrier to the species diffusion.