The spontaneous crystallization process of the planar zigzag form (form III) of syndiotactic polypropylene has been investigated at 0 degrees C for samples quenched from the melt by real-time wide-angle X-ray diffractometry and high-resolution solid-state C-13 NMR spectroscopy. The X-ray diffraction profile just after quenched from the melt is in good accord with that of the melt, indicating that the sample is in the noncrystalline state. With increasing crystallization time, broad diffraction peaks assignable to form III are found to evidently appear for the diffraction profile obtained after the subtraction of the noncrystalline contribution. The spontaneous crystallization of form III is also confirmed by high-resolution solid-state C-13 NMR spectroscopy; for example, the CH3(I) line assigned to the tt conformation really increased in intensity with increasing crystallization time in concomitancy with the decrease of the CH3(II) line ascribed to the tg conformation. The line shape analysis of these CH3 lines reveals that the degree of crystallinity rapidly increases up to about 0.10 in 5 h, but this value is still as low as 0.13 even after 4500 h. The additional crystallization of form I with the ttgg conformation is also found to be highly hindered for the sample kept at 0 degrees C for a longer period when it is annealed at room temperature, although this form is readily produced for the sample held at 0 degrees C for a shorter period. On the basis of these results, a structural model for the crystallization of form III at 0 degrees C is proposed by considering the possible formation of aggregates composed of the segmental chains with the trans-rich conformations in the noncrystalline region.