Abstract: A method for controlling the degree of branching of a 1,4-cis polybutadiene when preparing the 1,4-cis polybutadiene from 1,3-butadiene monomers in a nonpolar solvent using a catalyst system containing a an organonickel compound, alkylaluminum compound, and a boron fluoride complex is carried out by controlling the catalyst combination and a pretreatment condition. The method allows easy control of the degree of branching of the 1,4-cis polybutadiene without having to control the polymerization temperature or to add additional additives. As a result, the processability and the physical properties of the polymer can be optimized.