Abstract: An ultra-high strength aluminum alloy disc cracked near the edge after service for 1.5 months. The cracking cause of the disc was investigated by micro and macro observation, chemical composition analysis, local mechanical properties testing, finite element simulation (FEM) of in-service stress, and metallographic examination. The results indicate that the chemical composition near the crack meets the requirements of corresponding technical specifications, the local radial tensile properties near the crack meets the service requirements of the disc, and no obvious abnormality was found in the microstructure near the crack. Moreover, the heat treatment process of the disc meets the service requirements of the material. The fracture surface of the disc shows intergranular fracture feature. The cracking source is near the inner surface of the disc. The aggregation of oxides and ferric inclusions at the cracking source region is the main reason for the initiation and extension of the crack.