Abstract: During the work process of an oil well, the hanging sub joint of 27/8" Y8 changing 31/2" Y8 fractured. The fracture causes were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis and so on. The results show that the microstructure of the hanging sub joint was abnormal due to improper heat treatment, so early fatigue fracture occurred under the action of external stress.