The mechanical properties of aluminum alloys are strongly dependent on the thermo-mechanical process so, the strain ageing behavior of 7075 Al-alloy was investigated in this study. A set of test pieces was solution heat treated at 480 degrees C for 2 h, water quenched (SHTWQ) then pre-strained for 8% in tension. The test samples were aged at 140 degrees C for 0.5, 1, 2, 3, 4, 6, 8, 10, 12, 24, 48, 72 and 96 h in a furnace. The other set of test samples were solution heat treated at 480 degrees C for 2 h, quenched in sand (SHTSQ) then pre-strained for 8% in tension. They were also aged at 140 degrees C for same intervals. The effect of strain ageing, on the mechanical properties of Al-alloy, was investigated by mean of hardness, and tensile tests. The results shown that, the quenching rate after solution heat treatment, ageing time and temperature, as well as pre-strain, play a very important role in the precipitationhardening associated with ageing process of the 7075 Al-alloy.