AbstractBuckwheat has open‐flower morphology; therefore, isolation is necessary to propagate even in self‐pollinating lines. Here, we developed a self‐pollinating closed flower breeding line ‘CFK1’ by crossing between closed‐flowering individual and self‐pollinating breeding line. Progeny analysis suggested that the closed‐flowering trait would be dominated by double recessive gene, which we named closed‐flowering A and B (cfA and cfB). The evaluation test of outcrossing rate in the field revealed that ‘CFK1’ had 0% in progeny. An a histochemical investigation of flower buds morphology using scanning electron microscope revealed that the petals of ‘CFK1’ were tightly adhered to other petals whereas completely separated in wild type. The agronomic characteristics were evaluated by performance test. Compared to leading variety ‘Shinanonatsusoba’, the ‘CFK1’ had lower plant height, total weight, grain yield and 1‐L weight whereas higher plant height, total weight, grain yield and 1‐L weight when compared to ‘IH3’. The preharvest‐sprouting resistance of ‘CFK1’ is slightly stronger than that of other tested buckwheat. From these results, ‘CFK1’ is promising self‐pollinating breeding line that is resistant for outcrossing.