Abstract: For investigating the failure causes of pin joint fracture of a 139.7 mm heavy weight drill pipe,macro and micro analyses of the fracture surface were carried out.And the material chemical composition analysis,mechanical property test,metallography and finite element analysis were performed.The results show that the pin joint fracture of heavy weight drill pipe was corrosion fatigue failure.The tensile strength and torsion strength of the heavy weight drill pipe pin joint were reduced because its inner diameter was bigger than that specified in the standard,which led to the fracture.Under fatigue load and corrosion medium,corrosion fatigue crack appeared first at the bottom of thread which was the dangerous section part of the pin joint.Then the crack grew constantly under the loading and further reduced the joint strength,which led to final fracture.