Copolymerization of butadiene with styrene and diad analysis of the copolymers by ~3C-NMR were attempted to obtain information on the penultimate unit effect on the microstructure of the butadiene unit. As catalyst, Ni(acac) 2, Ni(benzoate) 2 and Co(acac) 3 were used together with Ett.sAICI~ 5. In the case of nickel catalysts, the cis content decreased with increasing styrene content in the copolymers; cis-rich copolymers were obtained and the trans unit competes with the cis unit. On the other hand, the cobalt catalyst produced copolymers containing a considerable amount of 1, 2 units, decreasing with increasing styrene content in the copolymers, while the cis content increased. These results suggest that the microstructure of butadiene unit seems to be strongly associated with the styrene unit incorporated. According to the ~3C-NMR spectrum of copolymer containing 21.2 mol°~ of styrene, the diad fraction of FsB having the cis unit was 45~o, whereas FBB having the cis unit was 91%. This means that the incorporation of the styrene unit diminishes the cis control of nickel catalysts. These findings are discussed under an assumption that a back-biting coordination of the penultimate butadiene or styrene unit onto the nickel catalyst is one of the most important factors for control of the microstructure of the butadiene unit in the copolymers.