Abstract: When downhole operated in an oil field, the N80-1 steel external upset tubing suddenly broke at the thickened part. Through geometric measurement, macro analysis, mechanical performance test, scanning electron microscopy and energy spectrum analysis, the reasons for the fracture of the thickened part of the tubing were analyzed. The results showed that the thickened part fracture of tubing was low-stress brittle fracture, and the main reason was that the casting defects of tubing blank changed internal stress state and stress distribution of tubing. The tensile properties of external upset tubing and outer diameter of tubing body did not meet standard requirements, the uneven wall thickness of thickened part, the rolling scar defect and the annular concave surface of tubing body were the secondary reasons for failure of external upset tubing. Under the combined effect of primary and secondary reasons, the tubing had a brittle fracture under the action of gravity that was much lower than rated minimum breaking force.