Common buckwheat has the sporophytic self-incompatibility mechanism and that’s why it has the ability to cross pollinate between two plants with different styles (thepin type and thrum type). The S supergene is thought to govern self-incompatibility, flower morphology and pollen size in buckwheat. Already, we have produced self-compatible buckwheat lines by an interspecific hybridization between Fagopyrum esculentum and F. homotropicum by embryo culture. The pollen size of F1 plants produced by a cross between a pin type plant and the self-compatible plant was similar to that of the self-compatible lines and segregated together with flower morphology without exception. The pollen tubes of the self-compatible plants were compatible with styles of the pin plants but incompatible with the styles of thrum plants. But, the pollen tubes of thrum flowers were compatible with the styles of self-compatible plants. Also, the pollen tubes of pin flowers were incompatible with the styles of self-compatible plants. Already, from these results, we have reported a tentative genotype for heterostyle and homostyle flower types. Homomorphism was controlled by a single allele Sh, while the pin/thrum-complex gene was governed by a single genetic locus S, with two alleles, S and s, which control Ss (thrum-type) as well as the ss (pin-type), respectively. Corresponding represents the case of a single locus S with three alleles, Sh, S and s, and the phenotypes, homomorphic, pin and thrum. It can be characterized by relationship of dominance, S>Sh>s. Using the two self-fertile lines, one is considered as the long-homostyle flowers and the other is considered as the short homostyle flowers. If the short-homostyle trait had arisen by recombination in the S supergene, its genotype would be considered to be GIs ip a/GIs ip a. The pollen tubes of the short-homostylous plant should be compatible with the styles of thrum plants. Also, the pollen tubes of short-homostylous plants should be incompatible with the style of long-homostylous plants, and the reciprocal cross also should be incompatible, because the genotype of long homostyle is gis Ip PA/gis Ip PA. Furthermore, the flower morphology of F1plants produced by the cross between cross and short homostyle flowers should be thrum or short homostyle and only short-homostylous plants should be produced by the cross between pin and short homostyle flowers. However, the compatibility or incompatibility of short homostyle flower was not clarified. So, we need to clarify the compatibility or incompatibility of the style of short homostyle flowers for the next step.