The effect of cultivation conditions on the optical purity ofL(+)-lactic acid produced by Rhizopus oryzae HZS6 from corncob hydrolysate was investigated.The isomeric composition of lactic acid was influenced by the supplementation of L(+)-lactic acid to fermentation medium.L(+)-Isomer increased with the dosage,no(-)-lactic acid was observed when the concentration of supplemented L(+)-lactic acid in mamx was ≥1.5g 1-1.However,the L(+)-lactic acid yield decreased with the dosage.Under suitable conditions,100g 1-1 initial comcob xylose,2g 1-1 NH4NO3,1.5 g 1-1 supplemented L(+)-lactic acid,R.oryzae HZS6 could produce 100%L(+)-form lactic acid with the yield of 75%and final concentration of 81.2 g 1-1,at pH 6.0 and temperature 34℃.