Abstract: A research on mechanics and high cycle fatigue properties of the third generation Al-Li alloy 2198-T8 has been carried out. Comparing with the same series Al-Li alloys with different heat treatments and other aluminum alloys used in airplanes, the material 2198-T8 shows a various improvement on tensile strength and yield strength and a better performance in ant-fatigue, but the dispersion of fatigue data increases with the decreasing stress level. In order to explore the internal mechanism of this phenomenon deeply, the advanced testing measurements including scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) have been applied. The results reveal that the propagation, the movement and the pileup of dislocations are the main reasons for fatigue crack initiation, and fatigue crack could be easily formed in the rough zone of persistent slip band (PSB ) and dislocation extrusion area. Under different stress states, the fracture caused by the material’s internal defect are fatal, having a huge impact on the fatigue life, which can reduce the fatigue life much more than 90%. Different crack initiation mechanisms can make the fatigue life changed greatly, which explains the problem of dispersion in fatigue data reasonably.