The interfacial structures of the iPP/PET matrix/fiber composite systems prepared by introducing the PET fiber into the iPP melt or supercooled melt were studied via optical and scanning electron microscopies. We found that unlike the iPP matrix/fiber single polymer composites prepared in the same way, where homoepitaxy induced by solid alpha-iPP fiber dominates the crystallization process of the iPP matrix, the iPP in iPP/PET composites displays the feature of crystallization in temperature gradient. Optical microscopy observations show that the interfacial structures of the iPP/PET composites depend strongly on the fiber introduction temperature. At higher fiber introduction temperature, e.g., over 170 degrees C, the crystallization of iPP takes place in its alpha-phase. At lower fiber introduction temperature, e.g., below 145 degrees C, the iPP crystallizes in its beta-phase. This is different from the crystallization of iPP in iPP/PET composite under quiescent conditions, where the PET exerts only a moderate alpha nucleating effect toward iPP and may provide a new way for the preparation of beta iPP.