Abstract: An investigation was given on the fracture accident of coupling on 114.3 mm tubing in one well by means of chemical composition analysis, mechanical property testing, macro and micro examination. The results showed that the tubing coupling fractured in use because there were original cracks in coupling which was produced in manufacture before copperizing. In order to prevent similar accident, the batch of tubing were rejected to manufactory for repairing, and the defects including crack, rolled flaw, folding defects and so on, should not be allowed in tubing after repair.