Temperature-dependent form II to I transition in isothermally crystallized low-molar-mass polybutene-1 has been investigated by means of fast scanning calorimetry. The as-crystallized sample possesses metastable form II crystals with a wide distribution of lamellar thickness and thus a wide distribution of the density of intercrystalline links between adjacent lamellae due to the high mobility of the chain segments within form II crystals and a limited chain length (low molar mass). Upon annealing the as-crystallized sample at different temperatures, the form II to I transition was found to occur in a wide temperature range irrespective of the glass transition temperature. The form I crystals transformed below glass transition possess a higher melting point than those transformed at a temperature close to the crystallization temperature. The results indicate that those lamellae with less or no intercrystalline links are not restricted by the vitrified amorphous phase. The mobility of chain segments in those well-crystallized form II crystals remains even at a temperature few tens of kelvin below the glass transition.