Abstract: Chemical composition analysis, mechanical property analysis, metallographic exmianation and scanning electron microscopy were used to investigate the fracture failure of repaired oil tubing thread. The results show that the fracture of repaired oil tubing thread at downhole was mainly due to under-deposit corrosion and chloride stress corrosion cracking. The black material covering on the outer surface of thread roots caused under-deposit corrosion. And chloride ions entered into the metal matrix through gaps, chloride stress corrosion cracking occurred under the tensile stress condition, finally resulting in the fracture of repaired tubing at threads.