The Najadaceae is one of families of the subclass Alismatidae. Although the systematic value of characters of seed coats has been appreciated in this family, further micromorphological study of seed coats is still essential to Chinese species. The present authors examined the micromorphological characters of seed coats of the Najadaceae from China using scanning electron microscope. The details about micromorphological characters of seed coats　of six species are given here. The results show that the external tangential walls of exotesta are covered with filamentous stripes or dense verrucae. The latter only exist in Najas ancistrocarpa and N. graminea. The cell of endotegmen is usually narrowly oblong, and endotegmen tuberculae on the internal tangential walls are found in all the six species studied, which is identical with the report of Shaffer-Fehre (1991a) for other species in this family. These characters are useful for taxonomic research at specific level. It is found for the first time that just like endotegmen tubercula, the sculpture patterns on the surface of seed coat,such as filamentous stripes and verrucae, are similar to those of the Hydrocharitaceae, indicating that they have important value for systematics of the Najadaceae. The evidence from  seed coat supports the viewpoint of Miki(1937) and Shaffer-Fehre (1991b) that the Najadaceae is closly related to the Hydrocharitaceae and should not be treated under the order  Potamogetonales.