Positively charged membranes having different charge densities were prepared. Ultrafiltration of aqueous amino acid solution was performed at various values of pH, concentration of amino acid in the feed solution, and operating pressure. The effect of charge density of the positively charged membrane on the solute flux was examined. Neither the volume flux nor the solute flux of amino acid at the isoelectric point was affected by the fixed charge of the membrane. On the other hand, both the volume flux and the solute flux in pH ranges below and above the isoelectric point decreased with increasing charge density of the membrane. According to the transport equations with respect to the volume flux and the solute flux, the permeation properties as determined from experimental data were successfully evaluated by considering the effective charge density of the membrane and the degree of dissociation of amino acid in pH ranges below and above the isoelectric point.