Abstract: Effect of natural aging on microstructure and mechanical properties of the 6061 aluminum alloy was investigated by tensile test and transmission electron microscopy. The results show that a few tiny spherical GP zones precipitated from the matrix of aluminum alloy after natural aging for one month. A great quantity of GP zones and lath-like β-Mg2Si equilibrium phase precipitated after artificial aging for 6 h. More precipitate phases were found in the alloy after natural aging for one month at first then artificial aging when compared with the alloy after direct artificial aging. With the prolongation of natural aging time, strength of the aluminum alloy gradually increased and elongation gradually decreased. The properties tended to be stable after one month, that the tensile and yield strength, elongation were 217.27 MPa, 130.78 MPa and 18.40%, respectively. Good comprehensive properties could be obtained after natural aging for one month and artificial aging for 4 h, that the tensile and yield strength, elongation were 273.72 MPa, 228.63 MPa and 11.16%. Comparing with the alloy after direct artificial aging, the tensile strength and yield strength decreased slightly, and the elongation increased by 11.6%.