The liquefaction of potato starch, catalyzed by lactic acid produced in a fermentation process, was carried out as a means for pretreating lactic acid fermentation feed. Liquefaction yield and molecular weight distribution of liquefied starch fractions were largely dependent on pH values of solution. The feed was effectively liquefied in lactic acid solution, of which pH value was less than 3.5, under sterilization conditions of 388 K and 30 min. Starch in potato or its waste was also hydrolyzed into smaller molecular weight fractions, compared to soluble starch commercially available. The potato starch liquefaction proposed is considered to be a useful method for pretreatment of lactic acid fermentation feed.