In this paper, nanoparticles of cobalt oxide (Co₃O are prepared at different temperatures C, C and C using the hydrothermal method. Cobalt nitrate hexahydrate: Co (NOH₂O is used as precursor and potassium hydroxide (KOH) is used as precipitating agent. Particle size is controlled using precursor concentration. It is also investigated in this research that particle size increases at high-temperature. Nanoparticles of size between (13.62–17.81 nm) are obtained using this technique (Hydrothermal method). SEM results provide nonuniform distribution of nanoparticles with sharp grain boundaries. Electrical characterization confirms the semiconducting behavior of the material as resistivity decreases with increase in temperature. Electrochemical measurements show detection of hydrogen peroxide H₂O₂ by nanoparticles of Co₃O₄.