Abstract: When an airplane was inspected that an inspector found the fuel leaked off from the supplying fuel pipe for engine. It was caused by the welding joint crack. The crack cause of the pipe was analyzed by macro analysis, fracture surface analysis, chemical analysis, metallographic examination. The results show that assembly is not precise between the supplying fuel pipe and the clamp band. The crack occurred in the rough welding root where stress concentration existed. The character of crack is fatigue. And the fatigue crack doesn’t penetrate the all thickness of the pipe, but it meets the welding micro-crack in the inner surface of the burn-through area. All these factors caused the pipe leaked off finally. Two welding craters isn’t good to the combination property of the part.