Abstract: The final superheater tube of an ultra supercritical boiler leaked. The causes of leakage on superheater tube were analyzed by means of macro observation, chemical composition analysis, mechanical property test, metallographic examination and energy spectrum analysis. The results show that there were intergranular oxidation and original inner fold defects in the material of HR3C stainless steel final superheater tube, which were not detected by water immersion ultrasonic detection. During operation, stress concentration and crack source generated at the defect, and the crack propagated along the defect under the action of alternating stress during rolling, finally resulted in tube wall leakage.