Abstract: The dynamic mechanical properties of butadiene isoprene copolymer prepared with lanthanide catalysts (Ln-PBI) have been studied using torsinal pendulum method. The content of cis-unit in Ln-FBI is high. The glass transition temperatures accord with those calculated from the FOX law for random copolymers. When the butadiene content in Ln-PBI is higher than 80wt% a new T_x transition was observed over the temperture range from 233K to 243K. The results of ¹³C-NMR and X-ray diffraction spectra indicated that both the length of butadiene segments and the crystallinity increase with increase of butadiene content when the later is above 80% in the copolymers. The existense of the T_X transition is related to the crystalization of long segments of butadiene in Ln-PBI.