Abstract: Longitudinal fracture occurred in high lock nut of 7075-T73 aluminum alloy during assembly. The causes of fracture on high lock nut were analyzed by means of macro observation, SEM analysis, energy spectrum analysis and metallographic examination, combined with production process and parts review results. The results show that there was alumina inclusion defect in the raw material, the cracks started at the alumina inclusion and expanded along the thickness direction under the assembly stress, which resulted in the fracture of high lock nut. Adding raw material defect inspection process and implementing NDT process strictly were recommended to avoid the recurrence of such failure.