In this study, the effects of fiber diameter, molecular weight of the matrix polymer, and interfiber spacing in glass fiber-reinforced polypropylene composites were investigated on the interfacial microstructure. The influences of the surface state of the fiber and the heat-treatment condition on the interfacial morphology and the spherulitic formation process in the matrix were also investigated. Consequently, it was found that both the fiber diameter and molecular weight of the polymer significantly influence the thickness of the transcrystalline layer. Also, as the interfiber spacing becomes smaller, the spherulites in the matrix polymer are not seen to be formed between the transcrystalline layers developed on the glass-fiber surface. In addition, the radius of the largest spherulites in the matrix polymer was found to be about the same as the thickness of transcrystalline region and to largely depend on the holding time at the crystallization temperature and cooling condition (or rate).