A series of poly(ethylene terephthalate) (PET) nanocomposites were prepared by in situ polymerization containing different amounts of surface treated calcium carbonate (CaCO3). As verified by SEM micrographs, the dispersion of CaCO3 nanoparticles into the PET matrix was homogeneous when the content of CaCO3 was below 5 wt.%, however, some aggregates existed at higher filler content. DSC study results showed that the crystallization became faster, and there was, also, a shifting of temperature of the crystallization peak to higher value by the addition of CaCO3 till 5 wt.%, this being evidence that CaCO3 can act as nucleating agent. From TG curves it was concluded that PET and the samples with different content of CaCO3 nanoparticles presented good thermal stabilities, since no remarkable mass loss occurred up to 360 degrees C (<0.5%). Moreover, from the variations of the activation energies, calculated from the TG curves, it was clear that nanocomposites containing CaCO3 had a higher activation energy compared to the pure PET, indicating that the addition of the nanoparticles could slightly increased the thermal stability of the matrix. (C) 2013 Elsevier B.V. All rights reserved.