In the present work, the crystallization of isotactic polypropylene by adding three different kinds of nucleating agents (pyromellitic dianhydride, NaCl, SiO2) has been studied. The glass transition temperatures were determined using DSC and DMA method. The crystallinity indices of isothermally crystallized polypropylene were derived from DSC and wide-angle x-ray diffraction measurements. The influence of crystallization temperature on the size of crystallites and the relative amount of thermodynamically less stable beta-crystalline form was estimated by the x-ray analyses.