Abstract: To find out the failure causes of S135 drill pipe in directional drilling crossing, the fracure morphology, chemical composition, mechanical properties and microstructure were analyzed and correspomding calculations were performed using fracture mechanics theory. The results show that the fracture style of the failed drill pipe was brittle fracture. The lateral quenching cracks were found on the outwall of upset section during the drill pipe manufacture,great stress concentration was formed at the crack tip during using. When the practical stress at the crack tip was larger than the critical stress of making cracks unstable propagation, the drill pipe failed.