A series of PET/MWCNTs nanocomposites were prepared by in situ polymerization using different amounts of multi-walled carbon nanotubes (MWCNTs). The polymerization of poly(ethylene terephthalate) (PET) was prepared by the two-stage melt polycondensation method. The values of the activation energy of the nanocomposites, as calculated with the Kissinger's and Ozawa-Flynn-Wall (OFW) methods, are larger than the ones of pristine PET. These values are 107.9 kJ/mol for PET-0% MWCNTS and 154.0 kJ/mol for PET-1% MWCNTs which is the larger value among all. Avrami plots present a linear portion, almost parallel to each other, which is followed by a deviation at larger temperatures. Straight fines are obtained from Ozawa plots only for PET-0.25% MWCNTs at least for three different heating rates. The dependence of the activation energy on the degree of conversion, from the Avrami, Malek and Ozawa plots, gives indications that for the kinetic description of the cold-crystallization of PET/MWCNTs cannot be used only one crystallization mechanism which obeys to Avrami equation. Only for the PET-0.25 one crystallization mechanism can be used at least for the major part of the crystallization conversion. So. in order to describe their crystallization mechanisms at least two mechanisms with different activation energies must be used. These two mechanisms maybe are taking part in different degree of crystallization conversions for every nanocomposite. (C) 2009 Elsevier B.V. All rights reserved.