Several polymer composites formed from nylon 6/CaCO3 with different ratios and particle sizes were prepared using modified and unmodified CaCO3 as inorganic filler. The modification of CaCO3 surfaces was carried out by adsorption of oleic acid from toluene. TGA and DSC measurements show that the glass transition temperatures reduced by the presence of inorganic filler, whereas the melting temperature did not influenced. In all prepared polymer composites, the presence of filler accelerates the degradation process of the polymer. Dielectric properties of the investigated samples were studied in the frequency range from 45 Hz to 1 MHz and at temperatures ranging between 50 and 200 degrees C. The presence of CaCO3 led to a remarkable decrease in the conductivity of nylon 6. (C) 2010 Wiley Periodicals, Inc. Adv Polym Techn 28: 257-266, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20165