The polymerization of butadiene (Bd) with Co(acac)(3) in combination with methylaluminoxane (MAO) was investigated. The polymerization of Bd with Co(acac)(3)-MAO catalysts proceeded to give cis-1,4 polymers (94 - 97%) bearing high molecular weights (40 x 10(4)) with relatively narrow molecular weight distributions (M-w's/M-n's). The molecular weight of the polymers increased linearly with the polymer yield, and the line passed through an original point. The polydispersities of the polymers kept almost constant during reaction time. This indicates that the microstructure and molecular weight of the polymers can be controlled in the polymerization of Bd with the Co(acac),NaO catalyst. The effects of reaction temperature, Bd concentration, and the MAO/Co molar ratio on the cis-1,4 microstructure and high molecular weight polymer in the polymerization of Bd with Co(acac)(3)-MaO catalyst were observed.