Abstract: Heterotrimeric G proteins are involved in multiple cellular processes in eukaryotes by sensing and transducing various signals. G protein signaling in plants is quite different from that in animals, and the mechanisms of plant G protein signaling are still largely unknown. Several recent studies have provided new insights into the mechanisms of G protein signaling in rice grain size and yield control. In this review, we summarize recent advances on the function of G proteins in rice grain size control and discuss the potential genetic and molecular mechanisms of plant G protein signaling.