Abstract: Several drilling tool fracture accidents took place during air drilling in one oil field. In order to know the cause of the accidents, an investigation is taken and the failure analysis is carried out for one broken drill pipe. The fracture analysis includes the macro-fractography and micro-fractography, the experimental analysis of the chemical components, the mechanical properties, and the metallography of drill pipes, and the stress conditions analysis of drill pipes in air drilling. It is considered that fatigue crack is early produced on drill pipes, because the vibration load of drilling tools in air drilling is more severe than that in mud drilling. Fatigue cracks easily occur under alternating vibration. Because the current wellhead detecting instruments can not be used to reflect the change of pump pressure produced after drill pipe piercement timely and accurately, effective measures can not be taken in time to prevent the expansion of fatigue cracks. As a result, drill pipe fracture accidents occur.