In this study, the variations of tensile strength and yield strength of Al-20% Si alloy were studied. Copper, magnesium and nickel as alloying elements added from 1% to 3% respectively. The temperature range was from room temperature to 350℃. The refinement of primary silicon crystal was treated with phosphorous addition. The results obtained are as follows: 1. Tensile strnegth and yield strength showed more increased strength in refining treated alloy than that of in nonrefining alloy at elevated temperature. 2. Tensile strength and yield strength were increased with the contents of copper. Tensile strength showed the maximum at 150℃, but yield strength was decreased with increasing temperature. 3. The effect of magnesium addition on tensile strength and yield strength showed the maximum at 1% addition and 150℃. 4. Tensile strength and yield strength showed a slight increase with the content changes of nickel and they were decreased with increasing temperature.