Abstract: The diversity and niche perspectives of aquatic plants are considered as key indicators of the ecological functions in a river ecosystem, and hence provide essential information for the management of rivers. Here we measured the diversity and niche of aquatic plants in different habitats in the Oujiang River, based on the field investigation from 2010 to 2013 and the previous reports. The results were shown below:(1) there were 181 species(including subspecies and varieties) that belonged to 78 genera and 41 families in the Oujiang River. Ten areal types were classified from these genera, in which the cosmopolitan accounted for the majority(43.59%), followed by the tropical and the temperate. The ratio of the tropical to the temperature was 1.75. Moreover, the relative ratio of the tropical genera to the same areal type nationwide(1.77%) was higher than that of the temperature(1.21%) in the Oujiang River. These suggested that tropical genera were further developed than the temperate type in the Oujiang River.(2) The 30 temporary plots were divided into six types of habitats, including river, swamp, farmland, ditch, pond, and floodpain. The species density was the highest(0.09 ind/m2) in the floodplains, followed by the swamps(0.07 ind/m2). The species diversity(Shannon-Wiener Index, H) gradually increased as the reaches became lower in the farmland and the pond(farmland:Hup=2.335 Hmid=2.634, Hlow=2.63; pond:Hup=1.468 Hmid=1.585 Hlow=2.632), but in other habitats the species diversity was the highest in the middle reaches. The emergent and freely floating plants were usually the dominant aquatic species, whereas the values of importance of the submerged and floating-leaf plants were lower.(3) The niche breadth of the emergent plants was generally higher than that of other life forms of plants, and submerged and floating-leaf plants usually had low niche breadth. The niche overlap of the same life-form plants was the highest(0.5) between the freely floating plants, and was lowest(0.26) between the submerged plants. The niche overlap of the different life-form plants was the highest(0.44) between the emergent and the floating-leaf plants, and was the lowest(0.16) between the submerged and the freely floating plants. The value of importance of Alternanthera philoxeroides was the highest in all the surveyed plots except for the swamps. Moreover, the niche overlap between A. philoxeroides and other species was generally higher than that between other species in the same plots. Therefore the alien species A. philoxeroides may alter the community structure of aquatic plants, resulting in a lower species diversity of native aquatic plants.