Blends of isotactic polypropylene and polyamide-6/clay nanocomposites (iPP/NPA6) were prepared with an internal batch mixer. A high content of the beta-crystalline form of isotactic polypropylene (beta-iPP) was observed in the injection-molded samples of the iPP/NPA6 blends, whereas the content of beta-iPP in the iPP/PA6 blends and the iPP/clay composite was low and similar to that of neat iPP. Quiescent melt crystallization was studied by means of wide-angle X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy. We found that the significant beta-iPP is not formed during quiescent melt crystallization regardless of whether the sample used was the iPP/NPA6 blend or an NPA6 fiber/iPP composite. Further characterization of the injection-molded iPP/NPA6 revealed a shear-induced skin-core distribution of beta-iPP and the formation of beta-iPP in the iPP/NPA6 blends is related to the shear flow field during cavity-filling. In the presence of clay, the deformation ability of the NPA6 domain is decreased, as evidenced by rheological and morphological studies. It is reasonable that the enhanced relative shear, caused by low deformability of the NPA6 domain in the iPP matrix, is responsible for beta-iPP formation in the iPP/NPA6 blends. (C) 2004 Wiley Periodicals, Inc.