Degeneration of embryo, endosperm and suspensor cells in hybrid embryo at 1 to 5 days after interspecific cross pollination in the genus Fagopyrum were studied by using a light microscope and transmission electron microscope (TEM) to determine the nature of the post-fertilization barriers of interspecific hybridization in buckwheat. In comparison with the self-pollinated or legitimate-pollinated embryo, interspecific hybrid embryos showed various post-fertilization ultrastructural abnormal phenomena such as failure of endosperm development, vacuolated embryo cells with degenerated endosperm, degeneration of embryo, endosperm and suspensor cells. Both endosperm and suspensor cells were degenerated. Their degeneration may lead to the loss of normal endosperm nourishing and failure of nutrient transport to the developing embryo and thus may trigger off the observed embryo abortion. Comparatively fewer abnormalities occurred in the hybrid embryo of the cross between F. cymosum and F. esculentum than in that between F. tataricum and F. esculentum. This indicates that the cross combination F. cymosum × F. esculentum may be considered as a breeding material in buckwheat hybridization. The deficiency and degeneration of endosperm that were observed at 2 to 3 DAP may lead to the degradation of hybrid embryo. Therefore, rescue of hybrid embryos at this critical stage (2-3 DAP) by ovule culture may overcome one of the main post fertilization barriers.