The effects of heat treatments in Al-Si-Mg alloys on the microstructure and hardness have been investigated by the optical microscope, scanning electron microscope(SEM), and Rockwell hardness tester. The materials of various compositions are melted in a vacuum induction furnace under argon atmosphere. Five different Al alloys are prepared from commercial purity aluminium, magnesium and Al-25Si alloy. Two types of aging treatments are performed: i) Isothermal aging of the specimens at $150^{circ}C$, $170^{circ}C$ and $190^{circ}C$. ii) Pre-aging of the specimens at $60^{circ}C$, $80^{circ}C$ and $100^{circ}C$, and followed by final-aging at $170^{circ}C$ and $190^{circ}C$. After the heat treatments, Rockwell hardness are measured with all the specimens.