A supercooled melt of isotactic polypropylene (iPP) was extruded through a capillary die. Polarized light microscopy (PLM), wide-angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC) were used to investigate the effects of the relatively weak wall shear stress (sigma(w)), extrusion temperature (T(e)), and crystallization temperature (T(c)) on the structure and morphology of beta-form isotactic polypropylene (beta-iPP). beta-cylindrites crystals could be observed by PLM in the extruded specimen even at a lower sigma(w)'s (0.020 MPa), and the beta-iPP content increased with decreasing T(e). Under a given T(e) of 150 degrees C, the increase in sigma(w) positively influenced the beta-iPP content. The DSC and WAXD results indicate that the total crystallinity and beta-iPP content increased when T(c) was set from 105 to 125 degrees C; the other experimental parameters were kept on the same level. Although T(c) was above 125 degrees C, the b-iPP content obviously decreased, and the total crystallinity continued to increase. On the basis of the influences of sigma(w), T(e), and T(c) on the beta-iPP crystal morphology and structure, a modified model is proposed to explain the growing of shear-induced beta-iPP nucleation. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 3255-3264, 2011