► Cu2+ substituted CoFe2O4 ferrite synthesized by sol–gel method. ► Characterization by XRD, AFM, dielectric spectroscopy and impedance spectroscopy, FT-IR, SEM, VSM techniques. ► Fe3++Cu2+↔Fe2++Cu3+ responsible for conduction mechanism. ► Cu doped CoFe2O4 nanoparticles exhibit room temperature ferrimagnetic behaviour. We report structural, electrical and magnetic properties of Cu doped Co ferrite nanoparticles prepared by sol–gel method. Analysis of structural properties has been carried out by using XRD and reveals that all samples have single phase cubic spinel structure. The permittivity (ɛ), dielectric loss (tanδ) and ac conductivity (σac) were determined at room temperature as a function of frequency. The results of dielectric properties indicated the normal Maxwell–Wagner type dielectric dispersion due to interfacial polarization. Complex impedance spectroscopic studies reveal that the conduction in these samples is through the grain boundaries. Room temperature magnetic hysteresis measurements infer that the saturation magnetization decreases with Cu doping which may be due to the difference of the magnetic moment of Cu and Fe ions.