The influence of Mg content on the microstructures and mechanical properties at room temperatures of A1-3.5Cu- （0.71-1.81）Mg alloys was studied. Precipitation phases in the alloys were identified by TEM and HRTEM. The results show that when Mg contents increase from 0.71 to 1.81 wt%, the precipitates are transformed from S, S＂, 2, and 0 phases to S and St phases, and f2 phase is first observed in A1-3.48Cu-0.71 Mg alloy with Cu/Mg mass ratio of 5 during the conventional aging heat treatment （190 C/12 h）. Regard to aging hardness effect of the tested alloys, the hardness of the alloys improves with the increase of Mg content, but the increases become slow when Mg content is greater than 1.35 wt%.
