Abstract: In this paper,the fatigue behavior of peak-aged 3J21 alloy was studied in material test system (MTS). The macrofracture and micro-fracture were analyzed by scanning electron microscopy (SEM). The results show that the fatigue crack growth of peak-aged 3J21 alloys is mainly transgranular extension with a small percentage intergranular extension. The fatigue crack initiation life of peak-aged 3J21 alloy is shorter, the fatigue crack growth track is more straight, and the fatigue crack growth rate is larger. The fatigue crack growth resistance is smaller, so that the fatigue life is the shorter. The fatigue fracture consists of fatigue source, fatigue crack growth area and instantaneous fractured area. In low growing area, peak-aged fracture presents rock candy pattern,in middle growth area, the long fatigue striations are not found beside some small individual fatigue striations,and in stantaneous fault zone,two crack, quasi-cleavage and dimple could be observed.