The nucleating ability of halloysite nanotubes (HNTs) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarized optical microscopy (POM) and scanning electron microscopy (SEM). HNTs are identified to have dual nucleating ability for alpha-iPP and beta-iPP under appropriate kinetics conditions. The formation of beta-iPP is dependent on the HNTs loading in the iPP/HNTs composites. The composite with 20 phr of HNTs is found to have the highest content of beta-iPP. Under non-isothermal crystallization the content of beta-iPP increases with decreasing of the cooling rate. The maximum-crystal content is obtained at cooling rate of 2.5 degrees C/min. The supermolecular structure of the beta-iPP is identified as beta-hedrites with flower-cup-like and axialite-like arrangements of the lamellae. Under isothermal crystallization the beta-crystal can be formed in the temperature range of 115-140 degrees C. Outside the temperature range, no beta-iPP can be observed. The content of beta-crystal reaches the maximum value at crystallization temperature of 135 degrees C. The formation of the iPP in the composites is correlated to the unique surface characteristics of the HNTs. (C) 2009 Elsevier Ltd. All rights reserved.