Crystal transformation behavior and structural changes for the planar zigzag form (form III) of syndiotactic polypropylene, which is spontaneously crystallized at 0 degrees C from the melt, have been investigated in the heating process by DSC, wide-angle X-ray diffractometry, and high-resolution solid-state C-13 NMR spectroscopy. The DSC curve for the sample containing only form III crystallites, which is crystallized at 0 degrees C for 5000 h, shows an evident endothermic peak at about 20 degrees C and an additional exothermic peak at about 100 degrees C prior to melting at about 150 degrees C. The endothermic peak, which is shifted up to about 40 degrees C by annealing at room temperature, is found to be ascribed to the crystal transformation from form III with the planar zigzag conformation to form II composed of isochiral helices with the ttgg conformation by wide-angle X-ray diffractometry. Moreover, the CH3 resonance lines of the dipolar decoupling/MAS C-13 NMR spectra measured at different temperatures for the sample with almost the same structure as for DSC and WAXD measurements are well resolved into the components with the tt and tg conformations for the CH2-CH(CH3)-CH2 bond. As a result, it is found that about 16% form III is allowed to be transformed into form II, whereas most of form III is melted gradually in the transition temperature region and very rapidly above that temperature. Such melting of form III should induce the crystallization of form I above about 60 degrees C, and the final degree of crystallinity of form I attains to almost the same level as for the original form III. This process is really reflected on the appearance of the exothermic peak at about 100 degrees C in the DSC curve described above. An annealing effect of form III crystallites has also been examined just below the crystal transformation temperature from form III to II.