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alloys containing 6.6%, 11.6% and 20% silicon were modified by metallic sodium or the special flux containing phosphorus, so called P flux developed in our laboratory. Alloying elements such as iron, manganese, magnesium, calcium and copper were added in Al-20%Si alloy in various quantities. From the above experiments the effects of modification and additional elements on the solidification in a shell mold were discussed.The results obtained are as follows :(1) Although Al-Si alloys were modified by metallic sodium or P flux there was little change in a cooling process of Al-Si alloys in a shell mold.(2) It was also found that the time required for solidification was slightly shortened by the modification and proportional to (volume/surface area)2~ for casting as far as this value was less than 0.3. The proportional constant was the largest for the A1-20°05i alloy and the smallest for the A1-11.6°/°Si alloy. (4) Primary crystallization and eutectic temperature falls were found in Al 6.6°0 Si and Al 11 .6°05i alloys modified with sodium. On the other hand the crystallizing temperature of primary silicon of Al-20°05i alloys treated with P flux was higher than that of ordinary Al-20%Si alloys.(5) Cooling velocity during primary crystallization of A-20°0Si alloys treated with P flux was considerably reduced by calcium addition and eutectic solidification time was shortened by copper or magnesium addition.(6) Because of significant ability of calcium and magnesium to coarsen primary silicon, addition of extremely small quantity of calcium or more than 2 % of magnesium disturbed the effect of P flux on refinement of primary silicon crystals considerably.