To obtain an inexpensive catalyst composition used for producing a syndiotactic 1,2-polybutadiene, having a high level of catalyst activity and stereoregularity and rich in flexibility.

This preferable composition comprising (a) an iron-containing compound, (b) a cyclic hydrogenphosphorous ester and (c) an organoaluminum compound in order to polymerize 1,3-butadiene is obtained. The use of a component deteriorating an environment such as carbon disulfide and a halogenated solvent is avoided by using the catalyst composition. The melting temperature of the syndiotactic 1,2-polybutadiene can be changed within the range of about 125 to about 175°C by changing the catalyst components and componental ratio thereof. The ability for enabling the melting point to change within the wide range is an extremely favorable one by using a simple catalyst composition. The syndiotatic 1,2-polybutadiene can be used as plastics or an additive for a usual rubber composition and further is capable of carrying out the crosslinking together with a usual rubber using a crosslinking binder.