Abstract: The fracture failure mechanism of a friction-welded drill pipe joint is analyzed by means of macroscopic analysis, physical and chemical property test of the material and both macroscopic and microscopic analyses of the fracture proper. The results demonstrate show that the failure mechanism is the kind of fatigue fracture. The end face of the external thread joint of the drill pipe is mechanically deformed due to collision with some thing, and at the same time, the thread guide surface also has some mechanical deformation during the process of make-up, which leads to the poor engagement state of the thread, and thus reduces the bearing capacity of the joint. During the operation, under the combined action of alternating torsion, axial tensile and compressive stresses, a fatigue crack is first generated at the bottom of the thread where the joint stress is concentrated. And then the crack continuously expands under the action of the alternating loads. When the crack extends to the internal pressure limit position, a washout occurs, while the crack keeps expanding towards both sides. Once the crack expansion gets the effective bearing area reduced to the bearable critical load, a transient fracture of the drill pipe joint happens.