In situ Raman spectroscopy bas been used to reveal the molecular behavior of isotactic polypropylene during melting and crystallization. The 400 cm(-1) band assigned;to C-C-C bending shows a blue-shift during heating, suggesting an increase in conformational disorder, whereas the 1330 cm(-1) band assigned to CH2 twisting shows a red-shift owing to an increase in the interchain distance. We suggest that interchain expansion proceeds even in the melt state, while-the chain conformation becomes sufficiently disordered at the melting point. During the cooling process, the peak shifts at given temperatures ate essentially the same as those during lieating, except for the supercooled region where the peak shifts show obvious hystereses. By extrapolating the linear correlation between the peak shift: of the 400 cm(-1) band and the characteristic length along the chain axis, the critical stem size for crystallization is estimated to be similar to 2.4 nm.