Mechanical properties of hypereutectic Al-Si alloys can be improved through refining its primary Si. In this study, modification of primary Si in hypereutectic Al-Si alloys was attempted by addition of P in the form of AI-Cu-P. The Al-Cu-P was manufactured by extrusion and heat treatment of Al and Cu-10wt%P mixed powder compacts. The microstructures of Al-Cu-P show that after heat treatment interdiffusion of Al and Cu-P occurs. The effect of Al-Cu-P on the primary Si refinment in hypereutectic Al-Si alloys were examined. The Al-Cu-P manufactured in this study can refine primary Si in short holding time with small amount of P contents.