Hormones are important signaling molecules regulating developmental processes and responses to environmental stimuli in higher plants. Rice endosperm, the portion of the seed surrounding the embryo, is the main determinant of rice grain shape and yield; how-ever, the dynamics and exact functions of phyto-hormones in developing endosperm remain elusive. Through a systemic study including transcriptome analysis, hormone measurement, and transgene-based endosperm-specific expression of phytohormone bio-synthetic enzymes, we demonstrated that dynamic phytohormone levels play crucial roles in the developing rice endosperm, particularly in regard to grain shape and quality. We detected diverse, differential, and dra-matically changing expression patts of genes related to hormone biosynthesis and signaling during endosperm development, especially at early devel-opmental stages. Liquid chromatography measure-ments confirmed the dynamic accumulation of hor-mones in developing endosperm. Further transgenic analysis performed on plants expressing hormone bio-synthesis genes driven by an endosperm-specific pro-moter revealed differential effects of the hormones, especially auxin and brassinosteroids, in regulating grain shape and quality. Our studies help elucidate the dis-tinct roles of hormones in developing endosperm and provide novel and useful tools for influencing crop seed shape and yield.