SUMMARY Grain size is a pivotal factor that significantly influences grain yield. However, the genetic basis is mostly unknown. Here, we found that our previously identified wl1 mutant, which regulates leaf width development through the APC/C TAD1 ‐WL1‐NAL1 pathway, also exhibits a wide grain phenotype with increased cell expansion and proliferation in glume. Genetic analysis showed that the APC/C TAD1 ‐WL1 module also regulates grain width in a common pathway. Further, WL1 can bind to the regulatory regions of Narrow Leaf 2 ( NAL2 ) (a grain width positive regulatory gene) directly to repress its expression by downregulating histone acetylation levels of the chromatin to regulate grain width development. Meanwhile, we also found that WL1 participates in the cytokinin signaling pathway and regulates grain width by interacting with cytokinin B‐type response regulator RRB2 and inhibiting its transcriptional activation activity on A‐type response regulator OsRR6 . In summary, this study established a pivotal WL1‐mediated grain size regulation pathway, which is crucial for understanding grain development and improving crop yield.