Abstract: In the production of 1,2-polybutadiene by contacting 1,3-butadiene with a catalyst composed of (A) a cobalt compound, (B) an organic phosphine compound, (C) trialkylaluminum and (D) water in an amount of 0.25 to 1.5 moles per mole of said trialkylaluminum, the addition of (E) at least one compound selected from the group consisting of allyl halides, alkyl-substituted allyl halides, benzyl halides, alkyl-substituted benzyl halides, and tertiary aliphatic halides to the reaction system enables the molecular weight of the objective 1,2-polybutadiene to be regulated to any desired value without causing any change in microstructure and degree of crystallization of the product and also without deteriorating so much the polymerization activity.