The crystallization behavior of isotactic polypropylene (iPP) induced with various concentrations of the beta-form nucleating agent aryl amide derivative (TMB-5) at different temperatures was investigated with synchrotron small-angle X-ray scattering and wide-angle X-ray scattering (WAXS) and polarized optical microscopy. The WAXS results indicate that TMB-5 is a temperature-dependent selective nucleating agent for iPP crystallization. This means that only beta crystals or alpha crystals form at low or high crystallization temperatures, respectively; both beta and alpha crystals can be found at a suitable crystallization temperature. A mechanism was proposed to understand this phenomenon and the crystallization behavior of iPP with the nucleating agent. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 128: 628-635, 2013