Sufficient food supply is critically important to humans, and food security will be particularly challenging in view of the ever-growing world population, decreasing arable cropland, and global warming. To meet the challenges, most crop geneticists think it will be necessary to enrich the cultivated gene pool for breeding programs, and thus equally important to increase our understanding of the underlying genetic and molecular mechanism for grain yield. However, crop yield is a complex trait that is controlled simultaneously by multiple genes (i.e., quantitative trait loci [QTLs]) and heavily influenced by the surrounding environment; hence, to dissect such a complex trait into component contributory traits is necessary. Seed size (weight) is an essential yield component trait, and the past two decades have witnessed the identification of several hundred grain shape QTLs in rice; however, the genetic and molecular regulatory network of these QTLs remains largely unknown. Increasing lines of experimental evidence suggested that the plant hormone brassinosteroids (BRs) play a key role in seed size (weight) and hence crop yield regulation. Here, I briefly discuss the recent progress on how BR biosynthesis and signaling pathway have an impact on the important agronomic trait in crops.