Abstract: 2A12 aluminum alloy outer nut for aircraft oxygen pipe cracked during service. The cracking time and cause of the nut were analyzed by means of appearance observation, macro/micro fracture observation, metallographic examination and chemical composition analysis. The results show that the cracking of the nut is stress corrosion cracking, with the characteristics of bifurcating cracks and mud pattern corrosion products. The circumferential stress due to assembly is the main stress for stress corrosion cracking. The stress corrosion crack initiated from the front several buckles of the thread and the end surface. Deformation and wear of the end surface or thread fit wear resulted in local film damage and decrease of corrosion resistance, so stress corrosion crack initiated from there.