A detailed investigation on the thermally grown oxide (TGO) layers of EB-PVD thermal barrier coatings (TBCs) on as-coated samples and samples that were thermally exposed at 1050°C for up to 1013h was conducted by using transmission electron microscopy (TEM). It was observed that a mixed layer of alumina and zirconia exists in the as-coated condition. The thickness of this mixed layer remained constant over a long period of time. Thickening of the TGO during thermal exposure is due to the growth of a continuous layer of pure α-alumina caused by inward diffusion of oxygen However, a small amount of increase in thickness of the mixed layer was found in the samples exposed at 1050°C for more than 504h, which is predominantly induced by the aggregation of elements Cr, Co, Ni and the formation of spinels in the mixed layer.