This article describes the preparation process of two series of poly(trimethylene terephthalate) (PTT) nanocomposites with an addition of exfoliated graphite nanoplatelets with two different platelets' size (50 and 500 lm). The influence of their size on processing, physicochemical properties, morphology and, most importantly, electrical conductivity and barrier properties of thin polymer films has been studied. It was clearly found that smaller platelets enabled to obtain conductive thin polymer films with a nanoplatelet content of 0.3-0.5 wt.%. However, nanocomposite based on PTT with 0.5 wt% of EG with the flake size of 500 lm proved to be nonconductive. At the same time smaller EG platelets demonstrated a more uniform distribution in the PTT matrix, which was confirmed by means of scanning and transmission electron microscopies, thus giving the samples in question barrier properties with respect to CO2 and O-2. Moreover, it has been shown that both nanofillers did not have a significant influence on the phase transition temperatures and on the long period. They caused however a slight decrease of crystallinity which is an evidence of an antinucleating character of those nanoplatelets in the PTT matrix. (C) 2015 Society of Plastics Engineers