The synthesis of undoped cobalt oxide and cobalt oxide with 4%nickel doping is studied in samples obtained through hydrothermal technique. After the final heat treatment at 300°C, X-ray diffraction analysis indicated the formation of cobalt spinel oxide with a complete replacement of nickel in the lattice. An average crystallite size of ~42 nm and ~31nm was found, as well as a particle size of ~ 20 nm and ~30 nm for Co3O4 and Co3-xNixO4, respectively. Structural parameters were established through Rietveld refinement with a good correspondence between the simulated and the experimental pattern with values of x2 = 1.25 and GOF = 0.90 for Co3O4. The energy band gap was found through UV-Vis spectroscopy in two different regions: Eg1 and Eg2 values vary between 1.54 and 2.04eV for samples doped with nickel. The SEM results indicated the formation of nanostructures with semi-cubic shape and irregular rods. Parameters such as crystal size, particle size, surface area, as well as morphology of the final product depend on doping.