Nanocomposites composed of poly(ethylene terephthalate) (PET) filled with calcium carbonate particles of nanometer scale were prepared by polymerizing the polyester in the presence of the nanosized fillers. Besides plain calcium carbonate, carbonate nanoparticles coated with stearic acid were also used, in order to improve the compatibility between the polymeric matrix and nanofillers. Morphological analysis evidenced a good dispersion of both the nanopowders into the PET matrix, especially in the case of coated calcium carbonate. The strong interfacial adhesion between the two phases is also responsible for the increase of the glass transition and melting temperatures in the nanocomposites compared to plain PET. Finally, non-isothermal crystallization studies revealed that the coated CaCO3 is a good nucleating agent for PET. Analysis of non-isothermal crystallization data with the Ozawa theory was successful for plain PET and PET/un-CaCO3, but this method failed to describe the dynamic solidification of the PET/c-CaCO3 nanocomposite.