A polybutadiene rubber having an enhanced mechanical strength is produced in such a manner that the content of water in a solution of 1,3-butadiene in an inert organic solvent is controlled to 0.2 to 5 millemoles per liter of 1,3-butadiene; a first polymerization mixture is prepared from the controlled 1,3-butadiene solution, an organic aluminum compound of the formula Al R.sub.n X.sub.3-n, wherein R=C.sub.1-6 alkyl, phenyl, or cycloalkyl, X=halogen, and n=1.5-2.0, and a cobalt compound, for example, by aging a mixture of the controlled 1,3-butadiene solution with the aluminum compound for at least one minute and then by admixing the aged mixture with the cobalt compound; the first polymerization mixture is subjected to a cis-1,4-polymerization; a second polymerization comprising the resultant cis-1,4-polybutadiene, non-reacted 1,3-butadiene, the inert organic solvent, and a catalyst comprising an organic aluminum compound of the formula AlR.sub.3, a cobalt compound, and carbon disulfide is subjected to a 1,2-polymerization to produce a polybutadiene rubber consisting essentially of 5% to 30% by weight of a boiling n-hexane-insoluble fraction and 70% to 95% by weight of a boiling n-hexane-soluble fraction thereof and having an excellent mechanical strength; and after the 1,2-polymerization is shortstopped, the resultant polybutadiene rubber is isolated from the 1,2-polymerization mixture.