The influence of the aging treatment on microstructures and mechanical properties of 2099 Al-Li alloy were investigated by means of transmission electron microscopy, tensile properties measurement and slow strain rate test. There were one T6 temper and two T8 tempers in this study. The value of tensile strength in T8 tempers were both higher than that in T6 temper. With pre-deformation before the aging treatment, it increased the dislocation density, and therefore the number of T1 precipitate in matrix. Meanwhile, the duplex aging in T8 temper was equipped with better comprehensive property. In investigated of slow strain rate test of 2099 alloy, the loss of strength did not happen in this study. However, compared with samples tested in the air, the failure time rapidly decreased when samples tested in aqueous solution. The tensile strength, yield strength and elongation of alloy in T8 duplex aging condition were 590 MPa, 570 MPa, and 9.3%, respectively. 2099 alloy had excellent stress corrosion cracking resistance and no stress loss happened.