Abstract: In order to find out the cause of circumferential crack in weld zone of a 5" weighted drill pipe in a well, the macro analysis, chemical composition analysis, mechanical performance analysis, and crack micromorphology analysis were carried out for the weld samples of the failed drill pipe. The results show that the crack in the weld zone is a defect generated during the production process of the drill pipe welding seam. The crack don’t extend during service. The welding seam of the drill pipe is not annealed in time after friction welding to eliminate residual stress, and a small crack is produced at the root of flange. In the process of production transportation or machining, the flash is subjected to axial force, and the crack expands further.