Abstract The influence of molecular weight on the polymorphic behavior of polybutene-1 (PB-1) in cold crystallization from PB-1/iPP (isotactic polypropylene) blend has been investigated. Twelve PB-1/iPP blend samples of 90/10 (w/w) composition were prepared from three PB-1s and four iPPs with molecular weight of 77–711 kg/mol and 12–580 kg/mol, respectively, and their cold crystallization was studied by Fourier transform infrared spectroscopy, wide-angle X-ray diffraction and differential scanning calorimetry. The relatively small amount of iPP in the blend can facilitate form I′ crystallization and suppress form II formation of the PB-1, and it was found that by decreasing the molecular weight of either PB-1 or iPP, especially the latter, the content of form I′ crystals can be increased significantly. In PB-1 blends with iPP of the lowest molecular weight of 12 kg/mol, 90–95% of the crystalline PB-1 obtained was form I′. The observed molecular weight effects on the PB-1 polymorphism in the cold crystallization are attributed to miscibility between the PB-1 and iPP components in the molten state, and a size-constrained nucleation mechanism is discussed.