In fermentation-separation processes, nanofiltration membrane processes can be used to separate organic acidand other byproducts such as sugars and proteins. In this study, new nanofiltration membranes were prepared to improve organicacid permeability during the separation processes of fermentation broth. Positively charged nanofiltration membranewas introduced to reduce lactic acid rejection by electrostatic attraction between lactic acid and nanofiltration membrane.Newly fabricated nanofiltration membranes were prepared by grafting cationic polyelectrolyte, PEI, on membrane surface.Thenanofiltration membranes showed positively charged surface potential. As a result, lactic acid rejection was remarkablyreduced while the rejection of glucose was not changed significantly.