Through introducing the iPP fibers having different molecular weights into their supercooled or molten matrices, fiber/niatrix single polymer composites of iPP have been prepared, and the obtained interfacial morphologies were studied by means of polarized light microscopy. It was found that the interfacial supramolecular structure was affected by not only the introduction temperature but also the molecular weight of the iPP fibers. At low fiber introduction temperatures, solid iPP fiber induces the growth of alpha-iPP crystals and results in the formation of pure alpha-iPP transcrystallization layer in the vicinity of iPP fiber regardless of its molecular weight. At high fiber introduction temperatures, completely molten iPP fiber, with either high or low molecular weight, loses its nucleation ability toward iPP matrix. The interfacial structure induced by incomplete molten iPP fibers is different from fiber to fiber. This has been explained in terms of different relaxation behavior of the fiber, which has been further confirmed by the annealing experiment of the high molecular weight fiber/matrix system at the fiber introduction temperature.