Abstract: Fracture failure of a ZKG223 drill rod connecting casing pipe occurred during use.The causes of fracture were analyzed by macroscopical inspection,chemical composition analysis,metallographic examination and fracture analysis.The results show that the fracture type of the connecting casing pipe was fatigue fracture.Fracture originated from the thread damage of inner wall of the casing pipe.The loose connection between the casing pipe and the drill rod resulted in the screw impact and extrusion damage during service,which resulted in the initiation and propagation of fatigue cracks,and ultimately led to the fracture failure of the casing pipe.