This study comprises the analysis of the microstructure of injection molded specimens of polypropylene (PP) filled with SiO2nanoparticles. Three independent methods were used for morphology analysis: from the test specimens, thin sections were taken and examined in polarized transmitted light. Parallel samples were chemically etched and analyzed in reflecting light. By thermal analysis (DSC) in addition, the degree of crystallinity was determined along the wall thickness. The results clearly show five layers along the wall thickness. While the spherulite size decreases from the center to the edge continuously, thermal analysis exhibits a local maximum in the degree of crystallinity in the transition layer closest to the cavity wall. Generally both the spherulite size and the degree of crystallinity decrease as higher the filler loading.