The copper oxide (CuO) nanostructures of varying shape were synthesized by a simple hydrothermal method without using any template and surfactants. The influences of reactant concentration, hydrothermal temperature and time on the growth of nanostructure were investigated. And the samples were characterized by means of scanning electron microscope (FESEM) and X-ray powder diffraction (XRD). At the same time, photo-decomposition test and UV-vis spectroscopy were done. The results shows that the CuO nanostructure has monoclinic structure with single crystalline phase. However, the structure and morphology of CuO nanocrystal can be controlled by changing reactant concentration. Its photodegradation rate to methylene blue can reach 92.1%. And the optical band gap calculated of nanosheets is found about 5.89 eV. Moreover, the plausible growth mechanism for the formation of CuO nanostructure is proposed. (C) 2014 Elsevier B.V. All rights reserved.