In order to clarify effects of mold washing on matal penetration, the resistance to metal penetration of cold-setting silica sand mold with various zircon washes has been investigated by means of the laboratory apparatus which is capable of simulating metal penetration, and the wash layer of penetrated sand mold has been observed. It is shown that zircon wash is unable to prevent metal penetration and little particular change is recognized even if the wash layer is thicker. The reason for these is that cracks are generated in the wash layer when it comes into contact with molten steel and that molten steel penetrates into sand mold through these cracks.