The in situ microfibrillar blend of poly(ethylene terephthalate) (PET)/isotactic polypropylene (iPP) was fabricated through a slit die extrusion, hot stretch, and quenching process. The morphological observation indicates that while the unstretched blend appears to be a common incompatible morphology, the hot stretched blends present PET in situ fibers whose characteristics, such as diameter and aspect ratio, are dependent on the hot stretching ratio (HSR). When the HSR is low, the elongated dispersed phase particles are not uniform at all. As the HSR is increased to 16.1, well-defined PET microfibers were generated in situ, whose diameter is rather uniform and is around 0.6 similar to 0.9 mum. The presence of the PET phase shows significant nucleation ability for crystallization of iPP. Higher HSR corresponds to faster crystallization of the iPP matrix, while as HSR is high up to a certain level, its variation has little influence on the onset and maximum crystallization temperatures of the iPP matrix during cooling from melt. Optical microscopy observation reveals that transcrystalline layers form in the microfibrillar blend, in which the PET microfibers play as the center row nuclei. In the as-stretched microfibrillar blends, small-angle X-ray scattering measurements show that matrix iPP lamellar crystals halve the same orientation as PET lamella. The long period of lamellar crystals of iPP is not affected by the presence of PET micofibers. Wide-angle X-ray scattering reveals that the beta phase of iPP is obtained in the as-stretched blends, whose concentration increases with the increase of the HSR. This suggests that finer PET microfibers can promote the occurrence of the beta phase. (C) 2004 Wiley Periodicals, Inc.