Thin films of isotactic polybutene-1 (iPBu) with large lathlike flat-on form II crystals were prepared by isothermal crystallization from melt at 105 degrees C. The II-I phase transition of this kind of thin film was investigated by Fourier transform infrared spectroscopy, transmission electron microscopy, and atomic force microscopy (AFM). Spectroscopy results show that the phase transition starts immediately after cooling down to -10 degrees C. The phase transformation propagates quickly in the first 10 min but then slows down and reaches a plateau by further annealing at this temperature. Another quick transformation process has been realized by heating the related sample to room temperature. This has been associated with the nucleation and crystal growth processes of the phase transition and confirmed here by the morphological study. It has been confirmed by AFM that, at the early stage of II-I phase transition that happened at -10 degrees C, the formed form I crystals are too small to be visualized by AFM. With increasing aging time at room temperature, some form I iPBu crystals appear mostly at the edges of the flat-on crystalline sheet. These crystals grow further with time until the complete transformation of the individual whole sheet. This provides morphological evidence for the nucleation and crystal growth events during II-I phase transition of iPBu.