The nonisothermal crystallization behaviors of isotactic polypropylene (iPP) with an aryl amide derivative TMB-5 as beta-form nucleating agent has been investigated by differential scanning calorimetry, X-ray diffraction, and polarized optical microscopy. The feature of crystallite morphology depends on concentration and thermal conditions. At low concentrations, TMB-5 molecules aggregate into fibril. structures and presented blunt exothermic peak with a shoulder at high temperature. The surface of these fibrils host active sites tailored for the nucleation of beta-iPP, represented by clusters of microcrystallites. With increasing concentration, alpha beta-transcrystalline layer develops on the lateral surface of needle-shaped TMB-5. Enhanced multiple endotherms indicate the ensuing crystals are less perfect and easily transformed into more stable forms. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 314-325, 2009