Abstract: The early transversal cracking cause of a high temperature superheater channel relief valve stub was analyzed by the macroscopic analysis,microstructure examination,chemical compositions analysis and XRD analysis. The result show that the stub cracked from inner wall of the stub and fracture mechanism was brittle fracture. There was short-time temperature excursion during the stub working process,and then pearlite of the microstructure was spheroidized and carbide aggregated at the grain boundary,so tensile strength and elongation percentage of the stub material was deteriorated. There was binding force to the stub so it cracked through transverse direction. There was no obvious plastic deformation because of the large binding force,and the fracture showed brittle rupture.