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The effect of natural aging time on tensile properties and fracture of the heat-treated AA2024-O Al-alloy was investigated. The stress-strain curves, the yield strength and the tensile strength of the specimens with different natural aging time were obtained by carrying out tensile test at room temperature. The constitutive equation that could predict the stress was established. The effect of natural aging time on the material microstructure was analyzed by observing the fracture. The results of the experiment demonstrated that the specimens were gradually strengthened to a certain point, and then the yield stress and the ultimate tensile strength decreased until the material reached a stable state. There were many dimples distributed on the fracture surface that was observed by SEM, so the fracture surfaces were thought to be a ductile fracture. Despite natural aging during long periods, the specimens of dimples became fewer and the plasticity worsened. A series of phenomenological functions were presented for establishing the constitutive equation of AA2024-O Al-alloy strengthened. The model could predict the stress of the naturally aged material.