Polypropylene (PP) samples filled with different carbon blacks (CB) were prepared by the conventional melt-mixing method. The distribution of the CB in the PP matrix was investigated by using the RuO4 vapor-staining technique and transmission electron microscopy (TEM). Our results indicate that amorphous regions of PP are sandwiched between the crystalline lamellae. The thickness of individual amorphous layers is a few nanometers. The amorphous region was measured to be about 45 wt % using a differential scanning calorimeter (DSC). The thickness of the lamellae is much smaller than the dimensions of the CB particles, and the CB particles or aggregates are dispersed inside the spherulites. There is a thin layer of amorphous PP encapsulating the CB particles and aggregates.