The retention property of nanofiltration membrane was studied for a series of organic molecules. Molecular weight and Stokes diameter of organic molecules were varied in order to correlate the retention to the size of the molecules, which is the main factor of determining the retention. Four different membranes were used; among which NF 70, NTR 7450 and Zirfon have negative charge in neutral condition and UTC-20 has positive charge. A good correlation was found except the Zirfon membrane, where retentions were too low to obtain a reasonable correlation.