Modified polybutadiene having a high-cis and low-trans structure with a moderate 1,2-structure content and exhibiting improved characteristics such as cold flow properties, which is obtained by modifying starting polybutadiene having a Tcp/ML.sub.1+4 ratio (Tcp: 5% toluene solution viscosity at 25.degree. C.ML.sub.1+4 : Mooney viscosity at 100.degree. C.) of 2.5 or more in the presence of a transition metal catalyst.