The age hardening process for permanent mold samples of Al-7Si-0.3Mg cast alloy has been investigated by hardness measurement, differential scanning calorimetry (DSC), transmission electron microscope (TEM) and electron probe micro analyzer (EPMA). Age hardening results show that the age hardening response of Al-7Si-0.3Mg alloy is independent on cooling rate. There is a hardness value decrease about 10 HV after T4 treatment. Hardness value after as-cast aging at 150 °C for 20 h is just a little smaller than that after T6 treatment for permanent mold samples. The precipitation behaviors during T6 treatment and as cast aging treatment have been analyzed by DSC analyses. The hardness measurement results have been discussed by analyzing the precipitation behaviors and the Mg and Si concentration in α (Al).