Pure and Co doped zinc oxide nanopowders have been synthesized by sol–gel route. The obtained powders, after calcination in air at 550, 650, 750 and 850 °C, were consolidated and sintered using microwaves. The effect of sintering temperature on the density and electrical properties was investigated. The best characteristics are obtained using a sintering temperature equal to 1,075 °C. The powders and sintered samples are characterized by X-ray diffraction, microstructure images, density and electrical measurements. The studied nanopowders have: a density of 5.22 (g/cm³), a breakdown voltage of 446.5 V/mm and a coefficient of non-linearity of 11.48 for ZnO doped with 7 mol% Co, and a density of 5.19 (g/cm³), a breakdown voltage of 292.5 V/mm and a non-linearity coefficient of 11.62 for ZnO doped with 5 mol% Co. The XRD results indicate that pure and Co doped ZnO powders are solid solutions crystallizing in pure würtzite structure, and consisted of a mixture of nanoparticles with an average grain size between 36 and 210 nm. The grain size decreases with increasing Co concentration and reaches its lowest value at 7 mol% Co and increases with increasing temperature. On the other hand the increase of Co concentration leads to a decrease of the porosity and an increase of the density of samples sintered at 1,070 °C and leads also to an increase of the electric field (E) in the non-linear area. The samples of 5 mol% Co sintered at 1,075 °C, show that the non-linear coefficient α decreases by increasing of calcined temperature, therefore increases with decreasing of grain and particle sizes.