Abstract: The S135 steel grade drill pipe of a certain well suffered corrosion failure when the drilling distance was about 2 395 m. The fracture reason was analyzed by macromorphology and fracture morphology observation, microstructure analysis, and chemical composition, hardness, and corrosion product composition tests. The results show that the failure mode of the drill pipe was corrosion fatigue failure. The inner wall of the drill pipe had high oxygen content and stress concentration in the thickened transition zone of inner wall, resulting in severe pitting corrosion in the thickened transition zone. Fatigue cracks were initiated in the bottom of the pits and propagated, fainally leading to the failure of the drill pipe. It was recommended to use the drill pipe with anticorrosive coating of inner wall, add corrosion inhibitor to the drilling fluid to reduce the corrosion effect of dissolved oxygen on the inner wall, and optimize the well depth structure to reduce stress concentration.