The loading of co-catalyst is an efficient way to increase the activity of synthesized carbon-based photocatalysts in the energy and environmental applications. Herein, Co and Ni decorated on g-C₃N₄ was synthesized as a visible light active photocatalyst and characterized with XRD, FT-IR, BET, DRS, TGA, FESEM, EDX, and EIS techniques. From the characterization results, it was demonstrated that cobalt and nickel which were present in the structure of the nanocatalyst, were in the metallic form. The decoration of Ni and Co reduced bandgap energy of g-C₃N₄ and made the synthesized nanocomposite active under visible light. The operating condition of O₂ photocatalytic generation was optimized (193.75 lmol L^-1 O₂ productions using 0.2 g Co2Ni1/g-C₃N₄/rGO at 7 h without sacrificial reagents). The effect of sacrificial reagents (Fe(NO₃)₃, AgNO₃, and methanol) was investigated and Fe(NO₃)₃ showed the best results during water oxidation. The kinetics study indicates that the presence of cocatalysts could be helpful in reducing the apparent activation energy about 3.5 times. The photocatalyst represented acceptable stability (2% Ni and 3%Co leaching) and reusability up to six cycles. According to the obtained results, we can introduce Co2Ni1/g- C₃N₄/rGO as a potential powerful photocatalyst for water oxidation reaction under visible light.