Abstract: It is difficult to avoid porosity and pinhole defects in cast aluminum alloys, which is the main cause of fatigue failure. Through the investigation on the fatigue properties of cast aluminum gearboxs for high-speed trains, it is shown that the size, shape and position of porosity defects have a significant influence on the initiation of fatigue cracks. The porosity defects away from the specimen surface have a small effect on fatigue fracture while the porosity defects near the surface tend to become fatigue crack sources. The larger and more irregular porosity defects are, the more easily the fatigue cracks form. In addition, pinhole defects were also investigated. The results show that with the increase of the grade of pinhole defects by X ray detection, the fatigue limit under axial R=0.1 gradually decreases. When the grade of pinhole defects is 2~3, the fatigue limit is about 70 MPa, and when the grade of pinhole defects is 7~8, the fatigue limit drops to 50 MPa. In the actual production process, employing low-pressure casting technology, enhanced refining degassing, and increasing cooling rate can effectively reduce the formation of porosity and pinhole defects, so as to improve the fatigue performance of the products.