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The time-dependent degradation of first-stage vanes for gas turbines was investigated. The specimens were prepared from the operating facilities in the Ulsan combined-cycle plant. A microstructural investigation of the thermal barrier coatings (TBCs) showed that spallation was occurring at inner root area of vanes during operation. Microstructural comparison of coating surfaces showed that exposure to the environment increased the thickness of thermally grown oxide (TGO) and that the b-depletion region also increased with operation time. After spallation of the top coat, mechanical thinning and the development of fine grains by recrystallization at the bond coat also occurred. The substrates of welded and normal regions were compared and showed that the difference in aluminum diffusion rate and crack density resulted from chemical and mechanical differences in the two areas.