As for the solid-solid transformation from form II to form I in polybutene-1, the intercrystalline links in amorphous phase and the temperature difference between crystallization and phase transition were thought to be essential in previous studies for generating the internal stress to promote nucleation and growth of the phase transition. In this work, we report experimental evidence showing that such a transition can be accomplished without supercooling at the temperature of crystallization directly in a low molar mass sample where there are almost no intercrystalline links. This result reveals two significant aspects of the form II to I transition in polybutene-1 against conventional wisdom: (1) the nucleation barrier of the transition is very low and can be overcome by thermal fluctuation; (2) entanglements and intercrystalline links in high molar mass systems play a role of stabilizing the metastable form II via hindering the translational movements of chains in crystalline phase and slowing the relaxation of amorphous chains during phase transition. Direct formation of form I' from melt begins in the early stage of the phase transition and proceeds during the phase transition. The melting temperatures of these form I' crystals increase with the aging temperature. Moreover, the structure and melting point of transformed form I highly depend on the annealing temperature.