Spherical CuO was synthesized by a simple hydrothermal reaction with the yield as high as similar to 90%. The size of the CuO spheres can be facilely and selectively controlled to range from several hundred nanometers to a few micrometers simply by adjusting the concentration of the reactants. The CuO spheres consist of many small and uniform nanoneedles with tips about tens of nanometers in diameter. Interestingly, it was found that the initial concentration of reactants plays an important role in the formation of spherical CuO with different size. A growth mechanism of spherical CuO was also proposed based on the observed results. The cyclic voltammogrametry reveals that CuO nanospheres exhibit a remarkable electrocatalytic activity for the oxidation/reduction of H(2)O(2). This study provides a promising route for the facile and cost-effective synthesis of inorganic nano- and micromaterials, and the synthesized CuO shows great promise in electrochemical applications. Published by Elsevier Ltd.