Abstract: Fracture failure of one 6″ (165.1 mm) drilling collar used in an oil field occurred during drilling. The fracture causes of the drilling collar were analyzed from the aspects of macroscopic inspection, ultrasonic thickness measurement, magnetic powder inspection, chemical composition analysis, metallurgical analysis, and so on. The results show that the fracture mode of the drilling collar was fatigue fracture. The cracks initiated at the bottom of the thread. With the annular clearance enlarged, the cracks initiated and propagated due to the serious stress environment during drilling. Finally, the drilling collar fractured under the impact of the internal high-pressure mud.