Aluminum alloy A357 can be heat treated in order to obtain optimum mechanical properties. Moreover, SDAS (Second dendrite arm spacing) is an important structure feature in cast aluminum alloys. The effects of SDAS on the homogenization of Al-Si-Mg alloy are investigated through measurement of SDAS and hardness analysis. The results show that the homogenization time increases as the value of SDAS increases and the homogenization temperature decreases in the homogenization process. The homogenization kinetics equation is: . In addition, the hardness after homogenization treatment at 540°C for 6 hours is advanced with the reduction of SDAS.