The kudzu starch cake was made by using arrowroot starch and sweet potato starch with water of different hardness. After investigating the influence that water hardness had on the quality of the kudzu starch cake. The results were found. 1. A difference was not seen in the quantity for the ash of the kudzu starch cake raw materials of three kinds of marketing products. The sweet potato starch content of calcium was a little more abundant compared with the arrowroot starch. On the other hand, there was slightly less magnesium had in sweet potato starch. The sweet potato starch contained slightly more phosphorus and zinc. The content of iron influences the color of arrowroot starch. 2. Sweet potato starch made harder kudzu starch cake than the arrowroot starch when the water of the same hardness was used. 3. When the hardness of water is high, the color of kudzu starch cake shows brown areas. 4. In the sensory rating, there were a lot of people who felt kudzu starch cake made with water of about 100 in hardness was delicious.