The effect of gamma irradiation on the release of the soluble and insoluble corrosion products was investigated with carbon steel and austenitic Type 304 stainless steel at high temperature using a small experimental loop. It was found that gamma radiation enhances the release rates of insoluble corrosion products (crud) but does not enhance the release rates of soluble species under the experimental conditions. On the basis of the result, it was considered that the crud forms in the bulk water through the oxidation of ferrous ion by oxygen or radiolysis products.