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Abstract: A number of poly(ethylene terephthalate) (PET) nanocomposites were prepared by in situ polymerization using different amounts (0.5, 1, 2, 3 and 4wt%) of fumed silica (SiO2). The polymerization of PET was carried out by the two-stage melt polycondensation method. From DSC studies it was found that the melting point of the nanocomposites was shifted slightly to higher temperatures by the addition of SiO2 till 3wt% while for PET–4wt% SiO2 nanocomposite the melting point was reduced. As the amount of SiO2 was increased the crystallization became faster, and there was, also, a shifting of the temperature of the crystallization peak to higher values, this being evidence that SiO2 can act as nucleating agent. At higher content (3 and 4wt%) the temperature of the crystallization peak is lower than that of PET–2wt% SiO2 due to the formation of crosslinked macromolecules. The activation energy is calculated with the Friedman''s method. PET/SiO2 samples present lower activation energy compared to that of neat PET, except those of PET–4% SiO2, in which the activation energy have a maximum value for α =0.8 probably due to the second crystallization peak. Extensive crystallization studies by using Avrami, Ozawa and Malek methods verified that PET and its nanocomposites must be crystallized by two mechanisms with different activation energies taking place in different degrees of crystallization. [Copyright &y& Elsevier]