Degradation behavior of the microstructure and mechanical properties of a long time serviced GTD111 DS superalloy turbine blade was investigated. Rejuvenation heat treatment was subsequently applied using a treatment of hot isostatic pressing (HIP) followed by solution and double-aging treatments. The results demonstrate that during service, pore density increases significantly from blade tip to root; MC carbides decompose into M₂₃C₆ carbides and η phase; secondary γ′ precipitates undergo severe spheroidization and rafting, accompanied by the dissolution of tertiary γ′ precipitates. The overall microstructural degradation pattern along the blade longitudinal axis follows the order: tip>central region>root>tenon. Along the transverse axis, it follows the order: trailing edge>leading edge>suction side>pressure side. The microstructure degradation directly leads to a progressive reduction in ultimate tensile strength at 25 and 980 °C and stress rupture life at 980 °C/220 MPa from the tenon to the blade tip. After rejuvenation heat treatment, the microstructure and mechanical properties are markedly improved. Area fraction of microporosity at the blade tip is reduced, MC carbides are partially restored, and the sizes of secondary and tertiary γ′ precipitates decrease to approximately 0.5 μm and 59 nm, respectively. Ultimate tensile strength of the blade tip increases from 810 MPa to 1122 MPa at room temperature and from 388 MPa to 468 MPa at 980 °C; the stress rupture life (980 °C/220 MPa) is prolonged from 1.95 h to 11.31 h. After rejuvenation, all mechanical properties at the blade tip exceed those of the tenon region.