Abstract: The bursting reason of a final super-heater T91 steel tube in a power plant was analyzed through chemical composition analysis, mechanical property testing, macro and micro examination.The results showed that, the burst tube had characteristics of overheat operation for short term, obvious plastic deformation was happened, the microstructure of the material aged seriously, and the mechanical properties decreased significantly near the burst mouth.The reason of overheat operation in the burst tube was possibly that, high temperature steam was blocked by weld beadings of the joint between two different diameter tubes and oxide skin spalling from the tube wall accumulated at the elbow tube, thus overheat operation for short term was caused in the tube local wall.