The influence of two kinds of polymers (polystyrene (PS), acrylonitrile-styrene copolymer (SAN)) on the crystallization behavior of isotactic polypropylene (iPP) has been investigated by means of wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). The current experimental results indicated that these two kinds of polymers with low concentration, as special [beta-nucleating agents, can induce the beta-iPP polymorph during quiescent melt crystallization. The nucleating activity of SAN or PS significantly depends on its concentration, molecular structure, and thermal history of processing. The content of beta-crystal form increases with the increasing crystallization temperature or nucleating agent (SAN or PS) percentage, reaches a maximum value, and then decreases as the temperature or nucleating agent percentage further increases. Under the same crystallization condition, SAN is more effective than PS on inducing a higher level of beta-crystal form in iPP. Besides, variable temperature WAXD and POM experiments have been used to investigate the polymorphism and the crystalline phase transformation of iPP. It was proved that the beta-crystal form of iPP is a thermodynamically metastable structure and will gradually transform to a new kind of W-crystal form with the rise in crystallization temperature. Because of different structural characteristics, the polymeric nucleating agents exhibit nucleation and crystallization mechanism which differ from the traditional low molecular weight beta-nucleating agents of iPP.