Abstract: The leakage reason of T23 steel tube in inlet header of high-temperature reheater of a power plant ultra supercritical boiler was analyzed through macroscopic examination, chemical compositions analysis, hardness test and metallographic examination and so on. The results show that higher hardness of the T23 steel tube and weldment restraint stress led to cracking at the pipe connection fillet weld in inlet header, and the cracks propagation caused the steam leakage.