In recent years, photocatalysis of mixed-oxide semiconductors has been attracted interest. Accordingly, coupling ZnO with CuO performs high photocatalytic efficiency to the characteristic optical properties of the composite. In this work, the ZnO/CuO nanocomposites were synthesized with two different initial pH values in an acidic media through a simple one-step and cost-efficient chemical bath precipitation method. The temperature of the water bath was fixed at 90 °C for 2 h and the precipitate powder was dried at 60 °C. To alter the pH value of the solution, nitric acid was added dropwise and initial pH values were 1.5 and 4.5, respectively. The crystal phase structure of the samples was performed by X-ray diffraction analysis (XRD), indicating the formation of wurtzite structure of ZnO and monoclinic structure of CuO. Additionally, the morphological structure of the as-formed nanocomposite was studied by field emission scanning electron microscopy (FESEM). It was demonstrated that at pH = 4.5 and 1.5, ZnO nanorods/CuO nanoflakes and ZnO nanoparticles/CuO nanosheets were formed, respectively. For optical characterizations, diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectra were performed. The band gap energy of the as-prepared samples was calculated to be 3 and 2.9 eV with an initial pH of 1.5 and 4.5, respectively. Furthermore, PL data revealed that the sample synthesized in pH = 4.5 exhibits a significant decrease in electron and hole recombination rate compared with that of the sample fabricated in pH = 1.5. The decreased bandgap energy and improvement in electrons and holes recombination rate were related to a p-n junction between CuO and ZnO. Accordingly, the photocatalytic activity of the as-prepared samples was studied employing 2 mg/L methylene blue (MB) under the visible-light irradiation. Overall, the prepared sample at pH = 4.5 and pH = 1.5 demonstrated ~76% and ~66% photo-degradation efficiency of MB after 150 min, respectively. Finally, the kinetic study of the photo-decomposition of MB, using the synthesized samples demonstrated a pseudo-first-order kinetics model.