The nonisothermal crystallization of multiwall carbon nanotube (MWNT)/ isotactic polypropylene (iPP) nanocomposites was investigated. The results derived from the differential scanning calorimetry curves (onset temperature, melting point, supercooling, peak temperature, half-time of crystallization, and enthalpy of crystallization) were compared with those of neat iPP. The data were also processed according to Ozawa's theory and Dobreva's approach. These results and X-ray diffraction data showed that the MWNTs acted as a-nucleating agents in iPP. Accordingly, MWNT/iPP was significantly different from neat iPP: A fibrillar morphology was observed instead of the usual spherulites. (C) 2003 Wiley Periodicals, Inc.