Abstract: The tensile properties of 3J21 alloy at over-aged state on a universal tensile test machine at room temperature under atmospheric conditions were investigated.By using metallographic microscope and X-ray,the metallographic structure and physical phase of the alloy at over-aged state were analyzed,the microstructure of the specimens of solid solution state and over-aged state as well as deformation near the tensile fracture was observed under TEM and the tensile fracture was observed under SEM.The results show that the tensile strength and yield strength of 3J21 alloy at over-aged state is lower and its elongation rate is larger at room temperature under atmospheric conditions.The tensile fracture of 3J21 alloy is a dimple fracture at over-aged state.The distance between the slip lines is larger in the microstructure near the fracture of the alloy at under-aged state.The influence of over-aged state on the tensile properties of 3J21 alloy was discussed at room temperature under atmospheric condition.