Abstract: In this paper, the bulk crystallization behavior of cis-1, 4-polybutadiene polymerized with rare-earth catalyst (Ln-PB) has been studied by linear dilatometry, volumetric dilatometry, DSC, TBA, and x-ray diffraction techniques. The effects of molecular weight of Ln-PB on the crystallization rate, melting point, glass temperature, and specific crystal surface free energy has been investigated in a wide range of molecular weight and crystallization temperature. It has been found that the crystallization rate increases initially with increasing the density of chain entanglement networks, through a maximum, and declines. Thereafter using secondary nucleation theory and taking into account the effect of the entanglement on the crystallization, a crystallization kinetic equation of Ln-PB which is quite adequate experimental data could be derived.