Taking quality sweet potato varieties with high-yield starch as experimental material,the effects of different pretreatment of sweet potato on fermentations of L-lactic acid with high optical purity by Rhizopus oryzea Fp-6 were studied,and the enlarge tests were carried out in 50 L fermentor. While comparing sweet potato starch to corn starch as experimental material,the results showed that there were no differences on the yield of L-lactic acid and fermentation period. When the concentration of organic N was higher than 0. 25 g / L in the liquefied sweet potato solution,the lactic acid yield and the conversion rate of sugar were decreased significantly,while the biomass and the content of by-products( fumaric acid and glycerol) were increased obviously. However,inorganic N had the role to facilitate lactic acid fermentation. In the fermentation test with the liquefied sweet potato solution which removed a part of organic N,the results showed that the yield of L-lactic acid was 91. 3 g / L,and the conversion rate of sugar was as high as 81. 28%,and the fermentation period was 48 h in 50 L fermentor.