Homo- and copolymerizations of butadiene ( ED) and styrene ( St) with rare-earth metal catalysts, including the most active neodymium (Nd)-based catalysts, have been examined, and the cis-1,4 polymerization mechanism was investigated by the diad analysis of copolymers. Polymerization activity of ED was markedly affected not only by the ligands of the catalysts but also by the central rare-earth metals, whereas that of St was mainly affected by the ligands. In the series of Nd-based catalysts [Nd(OCOR)(3) : R = CF3, CCl3, CHCl2, CH2Cl, CH3], Nd(OCOCCl3)(3) gave a maximum polymerization activity of ED, which decreased with increasing or decreasing the pKa value of the ligands. This tendency was different from that for Gd(OCOR)(3) catalysts, where the CF3 derivative led to the highest polymerization activity of ED. For the polymerization of St and its copolymerization with ED, the maximum activities were attained at R = CCl3 for both Nd-and Gd-based catalysts. The copolymerization of ED and St with Nd(OCOCCl3)(3) catalyst was also carried out at various monomer feed ratios, to evaluate the monomer reactivity ratios as r(BD) = 5.66 and r(St) = 0.86. The cis-1,4 content in ED unit decreased with increasing St content in copolymers. From the diad analysis of copolymers, it was indicated that Nd(OCOCCl3)(3) catalyst controls the cis-1,4 structure of the ED unit by a back-biting coordination of the penultimate ED unit. Furthermore, the long range coordination of polymer chain by the neodymium catalyst was suggested to assist the cis-1,4 polymerization.