High-temperature stresses during seed development reduces rice grain yield and causes poor milling quality because of insufficient grain filling. We monitored the progress of seed enlargement and the fluctuation in ATP content during seed development. When the rice cultivars Nipponbare and Taichung-65 seeds were developed in a high-temperature condition, a shortage in the amount of ATP was detected in the early and middle stages of seed development. This observation suggested that high temperature caused the insufficient accumulation of storage substances leading to the formation of small seeds along with chalky or white core endosperm. The flo2 mutant, which is thought to lack a regulatory factor for storage starch biosynthesis, produced chalky endosperm that resembled the typical phenotype of the high-temperature ripening-grain. The flo2 mutant exhibited a shortage of ATP content in immature seeds, suggesting that the gene responsible for the flo2 mutation is also involved in ATP production during seed development. In contrast, Kinmaze seeds maintained sufficient ATP content and their features did not differ significantly between mature grains developed in the normal- and high-temperature environments. These findings suggest that Kinmaze has some trait that is involved in the tolerance to high-temperature stress.