Recently, thermal barrier coatings (TBCs) have been used in advanced gas turbine plants for improved performance. Usually, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating. According to several studies, the failure of the TBC is induced by thermal stresses due to the formation of thermally grown oxide (TGO) at the interface between the TBC and MCrAlY. Therefore, it is important to investigate the high temperature oxidation behavior of the interface. In this work, the TGO is characterized in detail. In particular, in order to clarify the role of the TBC top coating in regard to initiation and growth of TGO at the interface, a specimen with TBC and one without TBC were compared. In both specimens, the TGO had two different contrasting layers. One was alumina, and the other was a combination of chromium oxide, nickel oxide, cobalt oxide, and spinels (hereafter call mixed oxide). The TGO thickness of the specimen with TBC was thicker than that obtained without TBC. These specimens had different oxidation behaviors. It is thought that the reason for the difference in TGO thicknesses of both specimens is due to a difference in oxygen potential, as the oxide compositions in the mixed oxides were different. In case of the specimen with TBC, the mixed oxide consists of chromium oxide, nickel oxide, and cobalt oxide, separately. On the other hand, in case of the specimen without TBC, the mixed oxide consists mainly of spinels such as (Ni, Co)(Cr, Al)2O4.