Abstract: The high temperature superheater tube of a power plant boiler leaked during operation. The leakage causes of the high temperature superheater tube were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination and tensile test. The results show that the flow rate of cooling medium was too low due to the blockage of foreign matters. The temperature of tube wall rose suddenly in a short time, which exceeded the phase transformation temperature of 12Cr2MoWVTiB steel, and the strength of tube wall was greatly reduced. Under the action of internal steam pressure, the tube wall plastic deformation occurred, the tube diameter expanded and the tube wall thinned. Then the tube wall was shear fractured and leaked.