Abstract: In this paper, the floral biological characters, pollination mechanism andbreeding system of Najas oguraensis Miki were primacily studied. A typicalsubmerged plant the pollination of the species occurs totally in water. In 2-4 hoursbefore the release of pollen, the pedicel of male flower grows rapidly and makesanther forced out of the membranous sheath and bends outward. As pollen released,the pedicel is almost horizontal, which is beneficial for the released pollen to betransferred by water current.In addition, the reduced structure of flower and abundantstarch grains in pollen all shows that N. oguraensis Miki is highly adapted tohypohydrophily, and sexual reproduction in this species is considerably effective. Inany case of geitonogamy and outcross, the ratio of seeds reproduction is over 85%.The ratio of pollen grains to ovules (p/o) is about 2690?00, which suggests abreeding system of facultative autogamy. Although asexual reproduction in N.oguraensis Miki is relatively poor and presents only in the form of fragments, it isbelieved very important for the dispersal of the species. Moreover some characteristicssuch as perianth reduction and exine reduction were explained rationally, and therelationship between geitonogamy and outcross in N.oguraensis Miki was discussedfrom the viewpoint of breeding system.