Abstract: T23, T91 and TP347H steel tube samples were taken at different positions of the final boiler reheater of a 600 MW supercritical boiler in service of about 7.8×10⁴ h. Microstructures and tensile properties at room temperature and elevated temperature of these samples were analyzed, and the deterioration degrees of the microstructure and tensile properties were studied and compared. The results show that the microstructures of T23, TP347H, T91 steel tube samples presented obvious degradation characteristics with M₆C carbide,M₂₃C₆ carbide and Laves phase precipitated on grain boundaries, and the microstructure degradation grades were grade 4, grade 3.5 and grade 4, respectively. The tensile properties of T23 steel tube samples at room temperature and elevated temperature showed serious deterioration tendency, and did not meet the standard requirements. The tensile strength of T91 steel tube samples decreased significantly. The tensile properties of TP347H steel tube samples met the standard requirements. Under elevated temperature conditions, TP347H steel tube samples had good strength and structural safety, but ductility was significantly reduced.