The polymerization of butadiene (Bd) with the soluble and insoluble parts of the NiCl2-methyl-aluminoxane (MAO) catalyst was investigated. Both parts initiate the polymerization of Ed to give a high molecular weight polymer consisting of mainly cis-1,4-structure. The activity of the soluble pare for the polymerization is higher than that of the insoluble part. We presume that NiCl2 reacts with MAO to give a soluble alkyl-nickel complex that shows high activity for the polymerization of Ed.