Rice ( Oryza sativa L.), like other grasses, features a unique organ known as the spikelet within its inflorescence, comprising floret, lemma/palea, sterile lemmas, and rudimentary glumes. Regarding sterile lemma, the molecular regulation underlying its identity remains elusive. Here, we isolated the G1 locus for sterile lemma specification using an F 2 population developed by crossing between Nipponbare and LG7, a variety with a lemma like-sterile lemma ( lsl). An SNP (+323G/A) in G1 alleles causes a serine to asparagine (S108N) substitution within its ALOG domain, leading to the lsl phenotype. Mechanistically, we found that G1 transactivates the expressions of both OsMADS34 and TGW2, two genes known to regulate sterile lemma identity and grain size, through binding to the YACTGTW and CArG-box motifs within their promoters, respectively. Intriguingly, the variation in G1 does not disrupt the binding of G1 LG7 (allele from LG7) to both OsMADS34 and TGW2 promoters but affects its transactivation activity. Subsequently, we reveal that the transactivation activity of G1NIP (allele from Nipponbare) is further enhanced through interactions with either OsMADS34 or TGW2. Furthermore, we demonstrated that G1 specifies sterile lemma identity via OsMADS34 and controls grain size through TGW2. Our results reveal two transcriptional circuits ( G1- OsMADS34 and G1- TGW2) that are crucial for determining sterile lemma identity and grain size of rice, providing insights into genetic improvement for breeding programs. Summary statement The isolated G1 gene, carrying an SNP (+323G/A) in its ALOG domain that causes a serine to asparagine substitution (S108N), underlies the lsl phenotype. Mechanistically, G1 transactivates OsMADS34 and TGW2 regulators of sterile lemma identity and grain size by binding to specific motifs (YACTGTW and CArG-box) in their promoters. Although the variant G1 LG7 retains promoter-binding ability, its transactivation activity is impaired. In contrast, G1 NIP exhibits enhanced transactivation through interactions with OsMADS34 or TGW2. We further demonstrate that G1 regulates sterile lemma identity via OsMADS34 and grain size through TGW2, revealing two critical transcriptional circuits coordinating these traits and providing insights for genetic improvement in rice breeding.