Grain size is one of the important agronomic traits, including grain length, grain width, grain thickness, and length width ratio.A mature seed consists of the embryo, the endosperm and the seed coat (pericarp), which develops from the zygote, the fertilized central cell and the maternal integuments, respectively.Grain size is therefore coordinately controlled by the growth of maternal and zygotic tissues.The spikelet hull of rice is a unique organ of gramineous plants, which sets a maximum limit for the size of rice seed.At present, several signaling pathways that determine rice seed size by influencing the spikelet hull growth have been identified, including the ubiquitin-proteasome pathway, the G-protein signaling pathway, the mitogen-activated protein kinase signaling pathway, phytohormones and transcriptional regulatory factors.It is of great significance to transform the basic research of grain size into practical application for the improvement of crop yield.In this review, we summarized the research progress of rice grain size control, with particular emphasis on the genetic and molecular mechanisms of several newly identified regulators of grain size in rice.