Cobalt oxide nanoparticles were prepared using a precipitation-oxidation method at mild reaction conditions. The nanoparticles had spherical shapes and sizes that ranged from 4.6 nm to 19.4 nm as determined by TEM and XRD. XRD indicated that the nanoparticles consisted only of the Co₃O₄ phase. The nanoparticles’ size was controlled using different amounts of the oxidant, hydrogen peroxide. The size was controlled in the absence of capping molecules. The yield increased with an increase in the amount of hydrogen peroxide used. The cobalt oxide nanoparticles were used as catalyst for the Fenton reaction to degrade methylene blue dye by hydrogen peroxide oxidation. The catalytic activity increased with a decrease in particle size. A percentage degradation of 99 % in 30 minutes was obtained when the 4.6 nm sized nanoparticles was used as catalyst. No significant methylene blue degradation was observed in the absence of the catalyst.