Abstract: Through means of macro and micro fracture analysis, metallographic examination, chemical composition analysis, mechanical property test and so on, combined with structure and working condition analysis, the fracture failure causes of the drill-stem sub in an oil well were analyzed. The results show that the failure mode of the drill-stem sub was fatigue fracture, and the lower bending strength ratio of the sub and serious stress concentration at the root of the internal threads were main causes for the early fatigue fracture of the drill-stem sub. The service life of the drill-stem sub could be effectively enhanced by measures such as optimizing of the structure to reduce the stress concentration of the thread root, improving the machining quality to reduce the surface roughness of the thread, improving the heat treatment process to achieve ideal microstructure and so on.