The melting and crystallization behaviors of the skin layer in an injection molded isotactic polypropylene (PP) have been studied, mainly in comparison with those of the core layer and subsidiarily in comparison with those of a compression-molded PP and a nucleator (talc)-added PP. The skin layer contains about 5% crystals, which have a high melting point of up to 184 degrees C. They thermally vanish by melting once. The subsequent melting history will scarcely affect the melting behaviors. On the other hand, crystallization behaviors are strongly affected by the melting history. The skin layer crystallizes in a wide temperature range at high temperature. This tendency weakens with increasing melting temperature, approaching a constant and that of the core layer above 230 degrees C, which suggests that the memory effect of the residual structure of PP vanishes by melting above 230 degrees C. In explaining these experimental results, it is assumed that the residual structure substance is a melt orientation of molecular chains that works as crystallization nuclei and that the vanishing of the residual structure is nothing but a relaxation of the melt orientation.