Some improved rice (Oryza sativa L.) cultivars such as the new plant type (NPT) lines of IRRI or Japanese indica × japonica cultivars can attach a greater density of spikelets, and hence have higher yield potential (YP); however, low grain ripening means that their higher YP is not fully used. Our objective was to clarify why their grain ripening is lower than in other cultivars, and whether low assimilate supply to grains or insufficient sink capability of spikelets restricts grain ripening in these cases. IR65564‐44‐2‐2 (NPT) and Akenohoshi (indica × japonica) cultivars yield lower percentages of ripened grain (PRG) and have lower spikelet filling percentages (F%) than other cultivars in field trials. When plant density was halved during the grain‐filling period (GFP) to improve plant dry‐matter increase (DMI), PRG and F% in Akenohoshi increased, but scarcely changed in IR65564‐44‐2‐2 and other cultivars. Inferior assimilate supply to YP limited the PRG in most cultivars due to close relationships between PRG or F% and DMI/YP; however, in IR65564‐44‐2‐2 increased DMI/YP did not improve PRG and F%. Among the higher YP cultivars, F% in IR65564‐44‐2‐2 started to decrease earlier with accumulated temperature. When panicles detached during the early GFP were cultured in a complete nutrient medium for 1 wk, F% in cultured Akenohoshi and IR65564‐44‐2‐2 was significantly greater than that under field conditions. These results suggest that inferior assimilate supply is the main cause of lower grain ripening in Akenohoshi, whereas the earlier decrease in grain ripening, probably due to higher spikelet nonfertilization, is the factor responsible in IR65564‐44‐2‐2.