The micron-size flower-like metal oxide of copper/zinc decorated graphitic carbon nitride (g-C3N4-CuO/ ZnO) composite was successfully synthesized by solvent mixing assisted with the sonication method. The structural, functional groups, morphology, particle size, optical properties of synthesized g-C3N4-CuO/ ZnO composites were analyzed using various techniques such as XRD, XPS, FTIR, SEM, TEM, UV-Vis, photoluminescence (PL), and photocurrent-time (PT) measurements. Further, the photocatalytic capacity of the synthesized composite was examined by the evolution of hydrogen production through the water-splitting using visible light and the study of methylene blue dye (MB) degradation under solar irradiation. The g-C3N4-CuO/ZnO composites showed the enhanced photocatalytic activity as compared to the pure bulk form of g-C3N4, DMSO-g-C3N4, and CuO/ZnO. Also, it showed an increment of photo -catalytic activity in the treatment of pollutants as well as increased production of hydrogen gas under visible light. These enhanced photocatalytic activities could be attributed to the combined effect between g-C3N4 and CuO/ZnO crystalline phases. The maximum quantum efficiency and degradation of the MB dye by g-C3N4-CuO/ZnO composite were 5.5% and 99%, respectively. The mechanism of enhanced photocatalytic activity of the g-C3N4-CuO/ZnO composite was also discussed in this work. (c) 2020 Elsevier Ltd. All rights reserved.