Nanocrystalline Ni-substituted cobalt ferrite sample is prepared by chemical co-precipitation method. X-ray diffraction and scanning electron microscopy techniques are used to obtain structural and morphological characterizations. Nanocrystalline nature is clearly seen in SEM picture. Variation of electrical resistivity as a function of temperature in the range 300-900 K is investigated. In rho versus 1/T plot shows four break resulting into five regions in 300-900 K temperature range of measurements. The magnetic transition temperature of the sample is determined from resistivity behavior with temperature. The activation energy in different regions is calculated and discussed. Variation of dielectric constant (epsilon') with increasing temperature show more than one peak; one at around 773 K and other around 833 K, which is unusual behavior of ferrites. The observed peaks in epsilon' variation with temperature show frequency dependence. Electrical and dielectric properties of Ni0.4Co0.6Fe2O4 sample show unusual behavior in the temperature range 723-833 K. To our knowledge, nobody has discussed anomalous behavior in the temperature range 723-833 K for Ni0.4Co0.6Fe2O4. The possible mechanism responsible for the unusual electrical and dielectric behavior of the sample is discussed. (c) 2007 Elsevier Ltd. All rights reserved.