Crystallization behavior of poly(ethylene terephthalate)/multiwalled carbon nanotubes (PET/MWNTs) composites have been investigated under isothermal conditions and in comparison with the conventional nucleating agents, sodium benzoate, and micrometric carbon/glass fibers. In the PET/MWNTs composites, MWNTs promote the crystallization of PET as a heterogeneous nucleating agent, and the nucleation efficiency is greatly enhanced when MWNTs was homogeneously dispersed in PET matrix. In comparison with pure PET, spherulites size of PET/ MWNTs composites is significantly reduced, and the shape becomes quite irregular. TEM images indicate that MWNTs bundles locate in the center of spherulites of PET and act as nuclei. Fold surface free energy during nucleation process for MWNTs nucleated PET is just half of pure PET, suggesting that MWNTs are efficient nucleating agents for PET. The sequence of nucleating ability of is given as follows: sodium benzoate>MWNTs>talc>carbon fibers glass fibers. The nucleation in the presence of sodium benzoate is a chemical nucleation process that may cause severe degradation of PET, but MWNTs nucleate PET through "particle effect," which does not affect the molecular weight of PET. (C) 2008 Wiley Periodicals, Inc.