CuO/ZnO composites are synthesized using a simple mechanochemical combustion method. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared (FTIR) are used to characterize the prepared oxides. X-ray diffraction reveals that the prepared CuO/ZnO exhibit a wurtzite ZnO crystal structure and the composites are composed of CuO and ZnO. The strong peaks of the Cu, Zn, and O elements are exhibited in the EDX spectrum. The FTIR spectra appear at around 3385 cm −1 and 1637 cm −1 , caused by O-H stretching, and 400 cm −1 to 590 cm −1 , ascribable to Zn-O stretching. The photocatalytic performances of CuO/ZnO nanocomposites are investigated for the degradation of methylene blue (MB) aqueous solution in direct solar irradiation. The degradation value of MB with 5 wt % CuO/ZnO is measured to be 98%, after 2 h of solar irradiation. The reactive • O 2 − and • OH radicals play important roles in the photodegradation of MB. Mineralization of MB is around 91% under sunlight irradiation within 7 h. The photodegradation treatment for the textile wastewater using sunlight is an easy technique-simply handled, and economical. Therefore, the solar photodegradation technique may be a very effective method for the treatment of wastewater instead of photodegradation with the artificial and expensive Hg-Xe lamp.methyl orange. Kuriakose et al. [14] prepared CuO/ZnO nanocomposites using the carbothermal evaporation method and evaluated the photocatalytic degradation of methylene blue and methyl orange dyes under sunlight irradiation. Wang el al. [15] prepared MnO@MnOx microspheres through the solvothermal process and reported the degradation of levofloxacin under simulated sunlight irradiation. Nanocomposite photocatalytic technology can be considered a green technology and provides the advantages of abundance, including postpone electron-hole recombination, higher photocatalytic activity, and the ability to convert solar energy to chemical energy, which eventually realizes the solution of energy and environmental issues [16][17][18][19][20][21].ZnO is an n-type semiconductor, having a conductivity of about 10 −7 -10 −3 S/cm. It has a relatively large binding energy of 60 meV. However, the main disadvantages of ZnO are its fast electron-hole recombination rate and inefficient utilization of sunlight that lead to a reduction in photodegradation efficiency. The photodegradation performance of ZnO can be increased by modifying ZnO with transition metals [22]. Among the various transition metals, Cu-doped ZnO nanomaterials are of special interest due to the photocatalytic efficiency enhancement that creates defects in the lattice and reduces the recombination of photogenerated charge carriers [23]. Cu also provides plenty of advantages, such as low cost, more electronegativity than zinc, and a similar atomic size to that of zinc, and leads to better doping efficiency [24]. CuO is a natural p-type semiconductor with a narrow band gap, having a conductivity of 10 −4 ...