Abstract: A self-locking nut of 0Cr12Mn5Ni4Mo3Al stainless steel cracked during service. Macro and micro fracture morphology observation, surface energy spectrum analysis, microstructure and mechanical properties analysis and testing were conducted on cracked nuts. The results indicate that the cause of the nut cracking was stress corrosion cracking (SCC). The crack originated at the junction between the flange surface and the nut body, where the nut body was at a right angle to the flange surface and was prone to liquid accumulation. The surface treatment of the nut was passivation and MoS₂coating, its corrosion resistance was poor. In a corrosive and humid atmospheric environment, localized corrosion points were generated under the alternating action of dry and wet conditions. Cracks initiated at the corrosion points and propagated under the tangential tensile stress of the nut circumference until cracking occurred.