Abstract: A drill pipe joint puncture event occurred during drilling in a directional well. The cause of the puncture was analyzed by microstructure and fracture morphology observation, chemical composition and mechanical properties test. The results show that the drill pipe was subjected to great bending stresses during the service, leading to stress concentration at the root of the 18° elevator shoulder of the drill pipe internal thread joint. Moreover, there were circumferential scratches at the root of the shoulder. Therefore fatigue cracks were originated and propagated. During the drilling of the drill pipe, the bending stress increased, resulting in the increase of the crack growth rate. The propagated cracks pierced the entire wall thickness, resulting in the puncture. It was suggested that optimizing the drill pipe structure and strictly controling the drill pipe processing technique to avoid scratches and reduce the stress concentration at the root of the shoulder.