By a small Ar gas pressure difference between the mold inside and outside in a vacuum chamber Si wafers for solar cells were cast in a size of 50 x 46 x 1.0 mm(3). The graphite mold coated by BN or Si3N4 powder, which was to prevent the reaction of carbon with the molten silicon, was used. Without coating, the wetting and reaction of Si melt to graphite mold was very severe. In the case of BN coating, SiC was formed in the shape of tiny islands at the surface of Si wafer by the reaction between Si melt and carbon of the graphite mold. The grain size was about 1 mm. In the case of Si3N4 coating, there were some reactions between silicon and nitrogen at the surface of the wafer. The grain size was very small in about 50 similar to 100 pm order. The Si3N4 coating acted as nuclei for the crystallization of Si melt. The compound formed on the wafer surface could be removed by grinding with sandpaper.