The conformational changes of quenched isotactic polypropylene (iPP) during stepwise heating have been carefully studied by in situ Fourier-transform infrared (FT-IR) spectroscopy. Analysis of the corresponding spectra shows that the mechanism of the extension of short helix into long one happens. Furthermore, it is easier for short helix at 998 cm(-1) band to extend into long one than that of short helix at 973 cm(-1) band. Meanwhile, for the higher order regularity band at 841 cm(-1), the melting-recrystallization seems to occur. However, the number of the higher order regularity band at 1220 cm(-1) hardly remains change, which shows the limited increment of the higher order regularity band for quenched iPP in the whole heating process. In summary, two proposed mechanisms, the extension of short helix into long one and the melting-recrystallization of the long helix, happen during stepwise heating, which may give us new insights on the mesomorphic-to-monoclinic phase transition mechanism of quenched iPP from a molecular structure viewpoint.