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Chromium substituted Co-ferrite nanoparticles with general formula CrxCoFe2−xO4 (0≤ x ≤0.5) have been synthesized by using sol-gel technique. The samples were sintered at 900°C for 3hours. The effect of chromium substitution on dielectric properties of Co-ferrites is reported in this paper. The X-ray diffraction patterns indicate the formation of single phase spinel structure for all the samples. The particle size of all samples was calculated using Debye- Scherer's formula and it is in the range of 56-75 nm. The investigation on dielectric constant (ε'), dissipation factor(D) and AC conductivity(σac) was carried out at a fixed frequency 1kHz and in the frequency range of 100Hz to 1MHz at room temperature using LCR meter. The frequency dependence of the dielectric constant (ε') and AC conductivity (σac) show a normal dielectric behaviour. The frequency dependence of dissipation factor (D) of the chromium substituted cobalt ferrite samples displays a relaxation peak at certain frequency. This peak suggests the presence of relaxing dipoles in the Cr-Co nano ferrite samples. It is also observed that the broadening of the relaxation peak with chromium-content(x) in dissipation factor versus frequency curve is due to the strengthening of dipole-dipole interactions. The dielectric behaviour of the Cr-doped cobalt nano ferrites is explained based on Koop’s two layer model and Maxwell- Wagner interfacial polarization theory and Debye-relaxation theory.