A study was conducted of the tensile properties of 3J21 alloy at peak-aged state on a universal tensile test machine at room temperature under atmospheric conditions. Moreove, a metallographic microscope was used to analyze the metallographic structure and physical phase of the alloy at peak-aged state. By employing a scanning electron microscope a tensile fracture was observed. The results show that the tensile strength and yield strength of 3J21 alloy at peak-aged state is higher and its elongation rate is lower at room temperature under atmospheric conditons. The tensile fracture of 3J21 alloy is a dimple fracture at peak-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 peak-aged state on the tensile properties of 3J21 alloy was discussed at room temperature under atmospheric condition.