The effects of thermomechanical and duplex aging treatment on precipitation behavior were investigated for the 8090 aluminium alloy by tensile test, hardness test, plane-strain fracture toughness test and electron microscope. Both pre-aging stretch and duplex aging with pre-aging stretch were effective to homogenize the distribution of S` phase in this alloys. The tatter makes more homogeneous distribution of S` phase than that of the former, but the sizes of S` phase in both specimens are almost same. The size and distribution of δ` phase were not changed by thermomechanical or duplex aging treatment. The strength was increased by thermomechanical treatment, but the elongation was decreased. Duplex aging treatment couldn`t change the strength and elongation. Pre-aging stretch and duplex aging with pre-aging stretch have same effect on the strength and elongation. The increase of strength by thermomechanical treatment in 8090 alumunium alloy was caused by homogeneously precipitated S` phase.