A series of bimetallic complexes composed of lanthanide metallocene and trimethyl aluminum, Cp*(2)Ln[(mu-Me)AlMe2(mu-Me)](2)LnCp*(2) (Cp* = eta(5)-C5Me5, Ln = Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Tm, Yb, Lu), were prepared, and the ability to catalyze polymerization of butadiene in the presence of cocatalysts, AlR3 (R = Me, Bu-i) and [Ph3C][B(C6F5)(4)], was studied as a function of the 11 different lanthanide metal species and the kind of alkylaluminum in the cocatalyst. Both catalytic activity and microstructure of polybutadiene depended strongly on the kind of lanthanide metal and the trialkylaluminum. The Gd complex in combination with Al-i-Bu-3/[Ph3C][B(C6F5)(4)] was extremely active and gave perfectly 1,4-cis regulated polybutadiene at -40 degrees C. In contrast, the Ce complex with almost similar cocatalysts, AlMe3/[Ph3C][B(C6F5)(4)], yielded polybutadiene having a very high content of 1,4-trans microstructure (1,4-trans selectivity = 93.8%). With the Al/Ln ratio was 3-5, the polymerization systems, in which the alkylaluminum worked as an activator, showed somewhat living-like natures the M-n of the resulting polybutadiene increased in proportion to monomer consumption, while the molecular weight distribution stayed narrow throughout the reactions. In the presence of a large amount of monomer, molecular weight of polybutadiene was conveniently controlled by additional excess alkylaluminum, which functioned as a chain transfer agent.