Abstract: In this paper the fatigue behavior of over-aged state 3J21 alloy was studied by using MTS testing machine.The macrographs and micrographs of fatigue fracture were analyzed by scanning electron microscopy(SEM).The results show that the fatigue crack growth for over-aged state alloys is transgranular and the fatigue crack initiation life is short as the proportion of growth along intercrystalline increases.The crack track is straight,the fatigue crack growth rate is fast,the fatigue crack growth resistance is low and the fatigue life is short.The fatigue fracture consists of fatigue source,fatigue crack growth area and instantaneously fractured area of over-aged state 3J21 alloy.In low growth area,the fatigue fracture show sugar candy pattern.In middle growth area,there were not any long fatigue striations inspected,except some small individual fatigue striations.