Rice atypical HLH protein Oryza sativa BRASSINOSTEROID UPREGULATED 1-LIKE1 (OsBUL1) is preferentially expressed in the lamina joint where it controls cell elongation and positively affects leaf angles. OsBUL1 knockout mutant (osbul1) and transgenic rice for double-stranded RNA interference (dsRNAi) of OsBUL1 produced erect leaves with smaller grains， whereas OsBUL1 overexpressors and an activation tagging line of OsBUL1 exhibited increased lamina inclination and grain size. Moreover， OsBUL1 expression was induced by brassinolide (BL) and osbul1 did not respond to BL treatment. To understand the molecular network of OsBUL1 function in rice， we isolated a novel OsBUL1-interacting protein， LO9-177， an uncharacterized protein containing a KxDL motif， and functionally studied it with respect to the lamina inclination and grain size of rice. OsBUL1 COMPLEX1 (OsBC1) is a basic helix-loop-helix (bHLH) transcriptional activator that interacts with OsBUL1 only in the presence of LO9-177 forming a possible trimeric complex for cell elongation in the lamina joint of rice. Expression of OsBC1 is also upregulated by BL and has a similar pattern to that of OsBUL1 Transgenic rice plants expressing OsBC1 under the control of OsBUL1 promoter showed increased grain size as well as leaf bending， while transgenic lines for dsRNAi and/or expressing a dominant repressor form of OsBC1 displayed reduced plant height and grain size. Together， these results demonstrated that a novel protein complex consisting of OsBUL1， LO9-177， and OsBC1 is associated with the HLH-bHLH system， providing new insight into the molecular functional network based on HLH-bHLH proteins for cell elongation.