According to several studies, the failure of thermal barrier coatings (TBCs) is induced by thermal stress due to the formation of thermally grown oxide (TGO) at interface between TBC and MCrAlY (M indicates Co, Ni or these combination). Therefore, it is important to investigate high temperature oxidation behavior at the interface. In this work, the TGO at the TBC/MCrAlY interface was characterized in detail. The TBC specimens were thermally aged at 1000 °C to simulate the surface temperature of first rotating blades. After aging, the TGO formed at the interface had two different contrasting layers. One layer was alumina, and the other was a combination of chromium oxide, nickel oxide, cobalt oxide, and spinels (hereafter called mixed oxide). The thickness of the two oxide layers increased with aging time. In order understand oxide formation, specimens with and without TBC were compared using identical substrate and bond coating materials. The TGO thickness of the specimen with TBC was thicker than that without TBC, possibly due to a difference in oxygen potential. These specimens had different oxidation behavior as shown by the extent of oxide growth and the presence of different oxides in the mixed oxide. In case of the specimen with TBC, the mixed oxide consisted of chromium oxide (Cr2O3), nickel oxide (NiO), and cobalt oxide (CoO) that were formed individually. On the other hand, in the case of the specimen without TBC, the mixed oxide mainly consisted of spinels (Ni,Co)(Al,Cr)2O4. The influence of the existence of YSZ top coating for initiation and growth of thermally grown oxide at the TBC/ MCrAlY interface was clarified.