Rice grain filling is impaired by high-temperature stress during seed development. This event results in reduced grain yield and quality because of formation of smaller and chalky grains, and often causes problems in agriculture. Hightemperature stress causes disorders in primary metabolism pathways including ATP accumulation in the developing seeds. To determine the effect on the genes for ATP biosynthesis, we created the transformant rice plants in which expression of an F1-ATPase β subunit gene (AtpB) was reduced by its RNAi gene, and elevated in developing seeds by its overexpression using the GlutelinB4 promoter. The transformants, which showed a high level of ATP accumulation as well as abundant AtpB transcript in the developing seeds, showed acquired tolerance to high-temperature stress during seed development because the ratio of perfect grains without any chalky part was significantly increased. In contrast, transformants harboring the RNAi gene showed reduction of grain quality with increase of ratio of white-core and white-berry grains when they were developed in the normal growth condition. Most of these seeds never germinated, and the RNAi transgene was poorly inherited by the progenies. These results suggest that AtpB plays important roles in rice seed development and that its gene expression is strongly influenced by high-temperature stress during the grain filling stage.