In this work, FeNbO4 powders were prepared using the sol-gel method. The fine powder particles were pressed into pellets and sintered at temperatures between 500 and 1200 °C. The powder was studied by X-ray diffraction and Raman spectroscopy. The morphology of the grains was investigated by scanning electron microscopy. Heat-treatment of the particles results in higher crystallinity, larger grains, and a decrease in the porosity of the material.The dielectric properties were measured in the frequency range of 102–106 Hz, in function of temperature (200–370 K). In all samples the real (ε′) and imaginary (ε″) parts of the complex permittivity increase with increasing annealing temperature. The sample heat treated at 1200 °C shows the highest ε′, > 104 at 300 K. All samples show a dielectric relaxation phenomenon, analysed using the modulus formalism. The evolution of the ac conduction activation energy and of the activation energy associated with the relaxation mechanism, is directly related with the changes promoted by the heat treatment in the structure and in the morphology of the base powders.