Crystallization in polymer systems actually is a process that transfers the entangled melts into a semi-crystalline layered structure. Whether or not a chain disentangles may result in different crystallization mechanism. When compared to the crystal thickness (dc), the volume occupied by the chain in the melt, i.e., the radius of gyration (Rg), plays a very important role in polymer crystallization. When dc ≤ Rg, crystallization does not necessitate a chain disentangling. The entanglements are just shifted into the amorphous regions. However, as dc＞Rg, i.e., as the crystal thickness gets larger than the radius of gyration of the chain in the melt, it becomes necessary for a chain to disentangle. Then a change of crystallization mechanism occurs. Such change has been experimentally observed in the crystallization of poly(1-butene). A change in the crystal morphologies from spherulite to quadrangle, is seen via PLM, as crystallization temperatures increase.Even more, such a change is molecular weight dependent, and shifts to lower temperature as molecular weight decreases. There exists a jump of crystal thickness and crystallinity associated with morphological change, as seen via SAXS. A change of crystallization kinetics and crystallinity is further evidenced via dilatometry. The unique feature of P1b crystallization has been discussed based on the radius of gyration of chain in the melt (Rg), and very good agreement is obtained.