A high-pressure 1st stage IGT blade made of René N5 served for nearly 26000 hours was studied to figure out the evolution of microstructure and the degradation degree in different locations of the blade. The morphology and volume fraction of γ′ precipitates, the topological inverse degree, and the morphology and composition of carbides were analyzed. It has been confirmed that the volume fraction of γ′ precipitates and topological inverse degree are applicable to be quantitative microstructural characteristic parameters to characterize the microstructural degradation in the served blade. And the morphology of the γ′ precipitates and the precipitation of TCP can be employed as the qualitative microstructural characteristic parameters. In terms of these parameters, the microstructural degradation of the served IGT blade could be classified into 3 degrees: slightly, moderately and seriously degraded. These results are expected to established as solid foundation for the assessment of the degree of the microstructural degradation of single crystal IGT blades and also have practical significance in service safety.