Effect of Cu content on microstructure and properties of Al-2.5 Mg-x Cu-0.2 Si alloys are investigated in this paper. The microhardness of the alloy with Cu addition has an obvious rapid hardening due to the clustering of Cu and Mg at the early stage of aging. With further aging, the microhardness of the alloy increases again and reaches an obvious second peak due to the formation of S’ phase and GPB zone. The increasing amount of Cu content would result a significant increase in tensile strength and yield strength while its elongation and intergranular corrosion performance reduces. With the increase of Cu content, the intergranular corrosion performance of the alloy becomes worse. The alloy would have a reasonably well corrosion performance when the Cu content is less than 1.1%, however there will be a significant reduction when the alloy in containing 2.0%Cu. Based on these results, the alloy containing 1.1% Cu would have better mechanical properties and corrosion resistance.