In this thesis, the graphite-like C 3 N 4 nanoparticles (g-C 3 N 4 ) were synthesized using two precursors including urea and thiourea through the pyrolysis method. In the next step, the synthesized g-C 3 N 4 nanoparticles were metal-doped by transition metal salts of zinc (II), silver (I), gold (III), and copper (II). The prepared materials were characterized using FT-IR, FE-SEM, BET, AFM, XRD, solid-state UV, and zeta potential methods. The results showed that the immobilization process of the nanometal or metal nano-oxides on the surface of g-C 3 N 4 were successfully done. Further, the photocatalytic activity of g-C 3 N 4 nanoparticles, and metallated g-C 3 N 4 nanoparticles as nanophotocatalysts on the photodegradation of Direct Blue71, methyl orange, and K-2BP pigments was investigated under visible light in aqueous solutions. The results exhibited the excellent efficiency of the prepared nanophotocatalysts in the photocatalytic degradation process of pigments. Moreover, the comparison of the results showed that the nanophotocatalysts obtained from the urea precursor have a better efficiency and a higher reaction rate in the photocatalytic degradation of the pigments than those obtained from the thiourea precursor. Therefore, the prepared nanophotocatalysts from the urea precursor were used to optimize the effective factors of the photocatalytic degradation of methyl orange dye under the visible light. The optimized factors are including the photocatalyst amount, concentration and the pH value of the colored solution. Finally, the amount of the photocatalyst and concentration of the colored solution were optimized at 0.015 g and 5 ppm, respectively. The kinetic study of the photocatalytic degradation reactions of methyl orange showed that all degradation reactions are of first-order, except for the pure g-C 3 N 4 which is zero-order. Finally, the photocatalytic activity of the prepared nanophotocatalysts from the urea precursor was investigated for the photocatalytic degradation of methyl orange under the sunlight in the optimal conditions. The results showed that these photocatalysts have efficient photocatalytic activities under the sunlight in addition to visible light.