The in vitro production of inter-specific hybrid plants between common (ovules) and homotropicum (pollens) buckwheat plants was attempted by the in vitro cross breeding procedure previously reported. At the initial stage, no fertilization or plant regeneration occurred in vitro between the two species, although fertilization did occur in potted conditions to some extent. It was found that pollen tubes stopped on stigma, i.e. halfway to the ovules, under a fluorescent microscope. It was also found that boric acid or calcium ions stimulated pollen tube elongation on stigma and penetration into the ovules. Boric acid treatment remarkably improved fertilization and suggested that a barrier to the pre-zygotic stage was short elongation of the pollen tube. At the post-zygotic stage, the development of the fertilized ovules and immature embryos into young plants in vitro was stimulated with coconut water. In vitro born plants were regenerated from approximately 8% of the pollinated flowers and this figure was doubled in the field. RAPD analysis suggested in vitro born plants were hybrid plants between common and homotropicum buckwheat plants i.e. inter-specific hybrid plants. One life cycle took approximately 60-65 days in vitro and 6 life cycles were possible, thus, suggesting fast cross breeding in vitro. The hybrid plants could be multiplied by a micropropagation procedure which indicated quick line establishment from inter-specific hybrid plants was possible in vitro. The present results suggest that quick cross breeding can be promoted between different species, but in vitro cross breeding in general is discussed.