In this article, the structure of Al-18Si alloy was modified by thermal-rate treatment technique at 930 A degrees C based on the DSC result. The mechanical properties of Al-18Si alloy were improved remarkable by a complex technique with alloying and thermal-rate treatment. A new treating technique named as complex modificating technique was proposed, and the performance of this technique on Al-18Si-1.5Cu-0.6Mg alloy was investigated. The results show that primary Si can be refined when Al-P master alloy was added into the melt at 770 A degrees C after thermal-rate treatment. Compared with the conventional casting technique by which the melt of alloy was unmodified, better refinement effect can be obtained with the combination of alloying and complex modificating technique: the size of primary Si is decreased from 66 to 16 mu m, the tensile strength increased by 75.94% and the brinell hardness by 66.59%. Moreover, the mechanism of the complex modificating technique was also discussed.