Abstract: The molecular weight distribution of cis-1,4 polybutadiene obtained by polymerization with homogeneous Ziegler catalyst (Et2AlCl-CoCl2-4C5H5N) as well as heterogeneous Ziegler catalyst (i-Bu3Al-Til4) has been determined by means of fractional precipitation. The results show that the molecular weight distribution of both types of polybutadiene is not broad. With regard to the homogeneous catalytic system, the molecular weight distribution becomes narrower when the temperature of polymerization is increased, the monomer concentration lowered, the chain transfer agent added and the cobalt concentration decreased or the Al/Co ratio increased. As to the heterogeneous catalytic system, the homogeneity of molecular weight distribution increases with the lowering of the polymerization temperature and the decreasing of the Al/Ti ratio. But the monomer concentration and aging of catalyst give no evident effect. Molecular weight distribution obtained with heterogeneous catalytic system is narrower than that obtained with homogeneous catalytic system. The peak of differential distribution curve of the former lies on higher molecular weight side, while that of the latter on lower molecular weight side. This is probably due to the fact that the chain terminating rate in the heterogeneous catalytic system depends upon chain length, while that in the homogeneous catalytic system is independent.