Abstract: Piercing failure had occurred in some S135 drill pipes in the process of a well drilling.A series of tests,such as chemical compositions analysis,metallographic examination,mechanical properties test and so on,were done to the failure drill pipes.According to these studies,the failure model of the drill pipes was regarded as corrosion fatigue.At the same time,tensional-compressive fatigue test was done to small round samples according to the failure model,which proofed the scientificity of design coefficient in the design of the drill string and provided data support for the failure cause.The analysis results show that it was the stress concentration of corrosion pits which located in outer surface of the drill pipes that resulted in the initiation and propagation of fatigue cracks,and the piercing failure occurred when the cracks propagated through the whole wall thickness.