The effects of fiber diameter and molecular weight of the matrix polymer in glass fiber reinforced polypropylene composites have been investigated on the interfacial microstructure. The influences of the surface state of fiber and the heat treatment condition on the interfacial morphology and the spherulitic formation process in matrix have also been investigated. From this study, it has been found that both fiber diameter and molecular weight of the polymer significantly influence the thickness of the transcrystalline layer. In addition, the radius of the largest spherulites in matrix has been 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).