Abstract: For a boiler high temperature reheater with long term service after 46 000 hours in a 660 MW ultra-supercritical unit, researches were conducted on the microstructure and mechanical properties of the leaked TP310HCbN steel tube by means of metallography test, scanning electron microscopy, tensile property test and impact property test. Results show that, the aging grade of the leaked TP310HCbN steel tube microstructure after long term service at high temperature is 3-4, and the tube material presents a serious embrittlement property with a large mount of carbide precipitating at the grain boundary, which result in the insufficient toughness of the tube material. A large number of micro-cracks are formed on the surface of outer tube by impacting, after which, the tube cracks gradually expand and eventually lead to leakage with the increase of service time.