In the present research, g-C₃N₄/MIL-88B(Fe) nanocomposite photocatalyst was successfully synthesized by the solvothermal method. The morphology, crystal structure, chemical functionalities, and optical properties of the obtained nanocomposite were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and UV-Vis diffuse reflectance spectroscopy (DRS), respectively. According to the SEM and TEM images, spindle-shaped MIL-88B(Fe) nanostructures were synthesized with an average length and width of 2 and 1 μm, respectively. Furthermore, g-C₃N₄ nanoparticles with an average diameter of 30 nm were observed on the surface of the MIL-88B(Fe). Based on the XRD results, the presence of both g-C₃N₄ and MIL-88B(Fe) nanostructures in the prepared composite was confirmed. Also, the presence of functional groups of the MIL-88B(Fe) and g-C₃N₄ was determined by FTIR. Based on DRS analysis and Tauc's plot, the band gap energy of the prepared composite was measured as 2.1 eV, which indicated that the prepared composite could absorb light in the visible region. The degradation of organic pollutant methylene blue (MB) in the g-C₃N₄/MIL- 88B(Fe)+light+H₂O₂ system was investigated to evaluate the photo-Fenton activity of the prepared composite in comparison with light and light+H₂O₂ systems. The results showed that the presence of g-C₃N₄/MIL-88B(Fe) composite increased the degradation rate of MB pollutant under the photo-Fenton process by 8.1 and 2.8 times higher than the mentioned systems, respectively. Thus, MB removal efficiency reached 100% within 20 min of illumination. The superiority of the g-C₃N₄/MIL-88B(Fe)+light+H₂O₂ system can be attributed to the retardation of electron-hole recombination due to the presence of two nanostructures of g-C₃N₄ and MIL-88B(Fe) in heterojunction, which has led to an increase in the efficiency of the photo-Fenton reaction.