The possibility of substituting potato or sweet potato for the wheat, one of the raw materials for soy sauce, was studied by measuring the amylase and proteolytic activities of Koji. Also optimum conditions of Koji making were determined. It was found that substitution of up to 30% (starch content) of wheat content (15% of the total bean and wheat content) with potato yielded good qulity of soy sauce. Use of more than 30% potato yielded a Koji of low enzymatic activity. This was attributed to the high moisture content of potato. It ws also found that substitution of up to 50% (starch content) of wheat content(25% of the total bean and wheat content) with sweet potato yieldede a good quality of soy sauce. But the taste was inferior to the control (the soy sauce which was made with 50% of bean and 50% of wheat).