The effect of intercrystalline links containing tie molecules and entangled loops on polymorphic transition from form II to form I in polybutene-1 (PB-1) of different molecular weights has been investigated by differential scanning calorimetry and small-angle X-ray scattering techniques. The PB-1 samples were isothermally crystallized at a range of temperatures to develop metastable form II crystalline modification of different lamellar thickness, long spacing, and number of intercrystalline links in the amorphous phase. Tammanns two-stage crystal nuclei development method was applied to promote a faster polymorphic transition from form II to I; that is, form I nuclei were developed at the early low temperature stage, and form I crystals were formed at the later high temperature stage. At a given annealing condition, the transition rate in the high molecular weight sample is found to increase with the increase of the prior form II crystallization temperature, while it shows a negative correlation between transition rate and crystallization temperature in the low molecular weight sample. The results can be understood as follows. The long spacing of high molecular weight PB-1 is smaller than the radius of gyration of the chains in the melt, leading to formation of folded-chain crystals and high possibility of generating intercrystalline links. Higher internal stress induced by unbalanced shrinkage of amorphous and crystalline phases would be built up during cooling from higher crystallization temperature to the first temperature stage of annealing. For low molecular weight samples, the probability of forming intercrystalline links decreases with the increasing lamellar thickness at elevated crystallization temperature. For a low crystallization temperature where intercrystalline links could be found in both samples, the low molecular weight sample shows a faster transition for its higher chain mobility. These molecular mechanisms are discussed in this article.