Vacuolar invertase is involved in sugar metabolism and plays a crucial role in plant growth and development, thus regulating seed size. However, information linking vacuolar invertase and seed size in rice is limited. Here we characterized a small grain mutant (grain size on chromosome 2) that showed a reduced in grain size and 1000-grain weight compared to the wild type. Map-based cloning and genetic complementation showed that is responsible for the observed phenotype. Loss-of-function of resulted in grains of smaller size when compared to the wild type, while overexpression showed increased grain size. We also obtained a T-DNA insertion mutant of , which is a homolog of and generated double knockout ( ) mutants of and using CRISPR/Cas9. Genetic data showed that , that has no effect on grain size by itself, reduces grain length and width in the absence of . Altered sugar content with increased sucrose and decreased hexose levels, as well as changes vacuolar invertase activities and starch constitution in , , mutants indicate that and affect sucrose metabolism in sink organs. In summary, we identified as a positive regulator of grain size in rice, and while has no function on grain size by itself. In the absence of , it is possible to detect a role of in the regulation of grain size. Both and are involved in sucrose metabolism, and thus regulate grain size. Our findings increase our understanding of the role of and its homolog, , in grain size development and also suggest a potential strategy to improve grain yield in rice.