Abstract: A drilling team in an oil field had a drill collar fracture failure accident during air drilling process. The causes of fracture failure of the drill collar were analyzed by means of macro observation, chemical composition analysis, mechanical property test, hardness test, metallographic examination, micro observation and so on. The results show that the thread joint of the drill collar did not match the size of the drill collar. The internal thread joint was a weak link and fatigue fracture occurred during air drilling process. It is suggested to adopt thread joint matching the size of the drill collar and to add stress dispersion grooves at the joint of the drill collar, or to use double shoulder joint drill collar.