A process for preparing crystalline 1,2-polybutadiene by contacting 1,3-butadiene with a catalyst consisting of (A) a cobalt compound, (B) an organoaluminum compound, (C) water in an amount of 0.25 to 1.5 mole per mol of said organoaluminum compound, and (D) an organic phosphorus compound in a halogenated hydrocarbon solvent to polymerize the butadiene, characterized in that as said organic phosphorus compound there is used a compound represented by the formula: ##STR1## wherein R.sup.1 is an alkyl or aryl group, and R.sup.2 and R.sup.3 represent independently hydrogen, or alkyl or aryl groups. This process can produce 1,2-polybutadiene having a degree of crystallization of 25 to 50% in a high yield at a temperature of from 0.degree. to 30.degree. C.