This paper describes variations in the microstructure/composition and mechanical properties in plasma sprayed CoCrAlY coatings and a modified Rene 80 substrate of gas turbine blades operated for 21000 h under liquefied natural gas fuels. Substantial oxidation/carbonization occurred in near surface coatings of concave blades but not in convex coatings. Aluminum and nickel/titanium rich nitrides formed in concave coatings and substrates adjacent to the interface, respectively. Small punch (SP) specimens were prepared in order that the specimen surface would be located in the near surface and interface regions of the concave and convex coatings. In SP tests, brittle cracks in the near surface and interface coatings of the concave blade initiated at low strains up to 950 °C. The convex coatings had higher ductility than the concave coatings and substrate and showed a rapid increase in the ductility above 800 °C. Thus it is apparent that the oxidation/carbonization and nitridation in the concave coatings produced a significant loss of the ductility. The in-service degradation mechanism of the CoCrAlY coatings is discussed in light of the operating temperature distribution and compared to that of CoNiCrAlY coatings induced by grain boundary sulfidation/oxidation.