This article deals with the crystallization behaviors of original (prepared in a torque rheometer), DSC crystallization and mold crystallization (quenching and slow nonisothermal crystallization) of isotactic polypropylene (iPP) mixed with beta-form nucleating agent. The microstructure and thermal stability of these samples were investigated. The wide angle X-ray diffraction (WAXD) results indicate that fast cooling is favorable for beta-form iPP formation. With slower cooling rate and higher concentration of nucleating agent, the lamellar thickness and stability of crystal0s were enhanced. Polarized optical microscopy (POM) and scanning electron microscopy (SEM) both showed that rapid crystallized samples gave rise to tiny spherulites, whereas under slow crystallization condition, nucleated samples could be fully developed in the form of dendritic or transcrystalline structures, depending on the nucleating agent concentration. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1374-1382, 2011