Abstract: Distyly was clearly confirmed in Polygonum japonicum. Research showed that in addition to dimorphic stigma and anther heights, the sepals, stigmas, pollen size, number and quality of seeds are also dimorphic. Compared with the thrums, the pins had larger sepals, and the stigmas had fewer but higher papillae, producing significantly smaller pollen grains but in greater numbers. The seeds of the thrum and the pin were ovoid and had no beak but did have paracerebroid omamentations. The pollen grains of the flower were spherical and suborbicular, with irregular margins and scattered holes. The micromorphological characteristics of pollen and seed supported P. japonicum belongingto the Persicaria-type. Ants were the main visitors to the flowers. P. japonicum often occured as a single population with greater setting rate of pin. The adaption of P. japonicum through its breeding system characteristics are discussed.