The skin-core structures of three kind of injection-molded isotactic polypropylene samples were studied by X-ray diffraction. The bend

tensile and impact properties were also measured. The results show that Sample A

without nucleating agent

contains mainly a-form and has lower crystallinity and β-form crystal amount (kβ-value) in the skin than in the core. Sample β

with a-nucleating agent

only contains α-form. But Sample C

with β-nucleating agent

contains mainly β-form and has higher crystallinity and kβ-value in the skin. The mo-dification transition between a-and β-forms occurs inthe sections of Sample A and Sample C. The differences of a-and β-forms in mechanical behaviors are explained with ther struc-ture features.