Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni(0.2)Cd(0.3)Fe(2.5-x)Al(x)O(4) (0-0 <= x <= 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary oil the electrical properties of the Al doped Ni-Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume Suggesting that the conduction mechanism takes place predominantly through grain boundary volume ill the Studied samples. The variation of impedance properties with temperature and composition has been Studied in the frequency range of 120 Hz-5 MHz between the temperatures 300-473 K. The hopping of electrons between Fe(3+) and Fe(2+) as well as hole hopping between Ni(3+) and Ni(2+) ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan delta) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.