Abstract: T91 superheater tube of a power generation company bursted in use, optical microscope and scanning electron microscopy were used to analyze the bursting cause of tube. The results showed that the number of intragranular carbide particles in burst tube had significantly reduced, grain boundary carbide was distributed as continuous chain structure along the grain boundary, and its size increased evidently, which indicated that the bursting of superheater tube was due to microstructure degeneration which was caused by overtemperature service, and eventually leading to bursting. And overtemperature service was induced by the coolant flow rate decreased due to the stripping of oxide layer.