We conducted principal component analyses using the thirty two quantitative characteristics of Najas from South Korea to examine the morphological variation and diagnostic characteristics. As a result of our investigation and the morphometric analyses, each taxon could be identified using the leaf width, shape of the leaf sheath, number of teeth on the leaf margin, number of anther cells, number of areoles in each longitudinal row of seeds, and the ratio of the areole width to the length. Dioecious Najas marina was clearly distinguished from the other monoecious taxa (N. graminea, N. gracillima, N. minor, N. oguraensis, and N. orientalis) by the larger size of the stem, the leaf, the flower and the fruit. The monoecious taxa could be identified using vegetative characteristics except for N. minor and N. oguraensis, which were distinct from each other according to the locule number in the staminate flower.