The large superfamily of labdane-related diterpenoids is defined by the cyclization of linear geranylgeranyl pyrophosphate (GGPP), catalyzed by copalyl diphosphate synthases (CPSs) to form the basic decalin core, the copalyl diphosphates (CPPs). Three stereochemically distinct CPPs have been found in plants, namely (+)-CPP, ent-CPP and syn-CPP. Here, we used X-ray crystallography and cryo-EM methods to describe different oligomeric structures of a syn-copalyl diphosphate synthase from Oryza sativa (OsCyc1), and provided a cryo-EM structure of OsCyc1D367A mutant in complex with the substrate GGPP. Further analysis showed that tetramers are the dominant form of OsCyc1 in solution and are not necessary for enzyme activity in vitro. Through rational design, we identified an OsCyc1 mutant that can generate ent-CPP in addition to syn-CPP. Our work provides a structural and mechanistic basis for comparing different CPSs and paves the way for further enzyme design to obtain diterpene derivatives with specific chirality. Copalyl diphosphate synthases (CPSs) catalyze the crucial cyclization step in labdane diterpene biosynthesis to generate stereochemically distinct copalyl diphosphates (CPPs), however, the stereoselectivity within CPSs remains underexplored. Here, the authors report the 3D structure and oligomeric properties of syn-CPS from Oryza sativa, which originally produces syn-CPP but also ent-CPP after mutagenesis. [ABSTRACT FROM AUTHOR]