The morphological development of a special polymeric nucleating agent [acrylonitrile-styrene copolymer (SAN)] in the isotactic polypropylene (iPP) matrix in the process of injection molding has been investigated by means of wide-angle X-ray diffraction and scanning electron microscope. The Current experimental results indicate that the shear field, in combination with the temperature gradient, has great influence on the morphological distribution Of SAN in the process of injection molding. For injection-molded SAN/iPP specimens with higher SAN concentration (>= 4%), SAN assembles to many microspheres and disperses uniformly in the isotropic core region; while from isotropic core region to oriented skin region, these SAN rnicrospheres are gradually stretched into fibrils as a result of shear effect. On the contrary, for the specimens with lower SAN concentration (<4%)), only microspheres can be observed in the core region and the skin region. At the same time, SAN has been proved to be a kind of special P-nucleating agent. The addition of SAN into iPP helps enhances the crystallinity and the content of P crystal form of injection-molded specimen. The morphology and the distribution of SAN in iPP matrix have great influence on the SAN's nucleating activity, which will ultimately affect the final crystalline structures of injection-molded specimens. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 786-793, 2009