The aging hardening behaviors of Al-8Si-3%Cu (wt%) and Al-3Cu (wt%) alloys have been investigated. Samples were solution treated at 500 for 24 h followed by water quenching before aging. Hardness has been measured for quenched samples aging at 150°C. Strong age hardening occurs for Al-3Cu alloy and hardness increases by about 60% after peak aging. There is a hardness decrease in the early aging stage of Al-8Si-3Cu alloy and hardness increases by about 15% after peak aging. The age precipitation behaviors have been analyzed using DSC and TEM. Effects of microstructure characteristics on age precipitation and age hardening response of Al-8Si-3Cu alloy have been discussed.