Abstract: The final superheater tube in a subcritical unit boiler leaked during running. The leakage causes were analyzed by means of macroscopic analysis, chemical composition analysis, hardness test, metallographic examination, scanning electron microscope and energy spectrum analysis and so on. The results show that the superheater tube was in long term over-temperature operation state, and the carbides were clustered at grain boundaries and spherically distributed. The carbon content of the matrix decreased, and the hardness, high temperature yield strength and creep rupture strength also decreased. The creep rate was accelerated, and longitudinal creep cracks were initiated on the outer surface. Crack propagation led to fracture and leakage of the final superheater tube.