The conformational change of isotactic polypropylene (iPP) during the crystallization process has been carefully Studied by Fourier transform infrared (FTIR) spectroscopy. The experimental measurements show that the iPP melt system is stable when the persistence length of helical sequences is less than 12 monomer units. As soon as the helix length exceeds 12 monomer units, the 3(1) helix conformation extends quickly and then crystallization occurs. These results are discussed in terms of Imai's microphase separation theory. It can be found that the experimental observation agrees very well with Imai's theory.