Abstract: It has been shown that cis-1,4-polybutadiene obtained by rare-earth catalyst (Ln-PB) possesses a cis-1,4-content higher than 98%,and can be crystallized rapidly at low temperature. This paper presents our study on the relation of molecular weight and temperature with the crystallization behaviour of Ln-PB with dilatometer and electron microscope. A set of crystallization rate curves of Ln-PB is given in this paper. It is seen that the minimum rate of crystallization occurs in the range of _η=4.6-6.7×10⁵, while the maximum rate in the range of _η= 3.0-3.8×10⁶.The result has been interpreted in terms of the nucleus theory of crystallization. It is also shown that, for Ln-PB, there is a transition at about -20℃ from regime Ⅰ to regime Ⅱ as crystallization temperature decreases. The interfacial free energy of Ln-PB, σ_e, is to be about 17erg/cm², when _η<10⁶, which increases with increasing molecular weight when _η>10⁶.