A new technique has been developed for in situ study of the form II to I phase conversion of isotactic poly(1-butene) (iPBu) in real space. It can distinguish the nucleation and crystal growth processes unequivocally, especially in the early nucleation stage, which can hardly be followed by other traditionally used techniques owing to the limited amount of the transformed form I crystals. Moreover, the subsequent growth process of the form I crystals revealed by the time-dependent images taken during the transition process makes it possible to estimate the growth rate of the form I iPBu crystals during phase transition of ca. 1.65 nm/min at room temperature, which is about 500 times slower than the melt crystallization process. It has also revealed the slow-down crystal growth in the late transformation stage. All of these indicate that the technique is powerful in studying many aspects related to the II-I phase transition of iPBu.