Abstract: An investigation was made into the failure of a 101.6 mm×8.38 mm G105 drill pipe by chemical compositions analysis, mechanical properties test, metallographic examination and X-ray diffraction, and the stress of the drill pipe during drilling process was also discussed. The results show that the piercement of the drill pipe was corrosion fatigue. On the action of the corrosion mediator, oxygen corrosion firstly occurred on the drill pipe, and then Cl- accelerated the corrosion. Etch pits were generated on exterior surface of the drill pipe which result in stress concentration in bottom of the etch pits. The fatigue crack generated in bottom of the etch pits where the stress concentration were heaviest under alternate load, expanded and finally formed penetrated crack and came into being piercement under the scouring of high pressure drilling fluid.