Abstract: This paper reports our work on low-temperature transitions of random copolymer of butadiene and isoprene prepared with rare-earth catalyst.It is shown by experiments that the glass transition temperature, the crystalliza-tion rate and the maximum main crystallization value of these copolymers vary with monomer ratio in the copolymer.When the weight ratio of buta-diene/isoprene reaches 84/16, the copolymer becomes noncrystallizable rubber. The maximum crystallzation rate temperature, Tc_max, of polybutadiene prepared with rare-earth catalyst was obtained as-72℃ by extrapolation of the experimental data of copolymer with various monomer ratio,which has not been reported in literature. The glass temperature,T_g of polybutadiene prepared with rare-earth catalyst is-113℃.The ratio of Tc_max(°K)/T_g (°K) is equal to 0.796,and that of natural rubber equals 201/247=0.814,both are close to 0.80.