Fe³⁺-doped g-C₃N₄ and Fe₂O₃/g-C₃N₄ composite catalysts were prepared by a simple method using melamine and ferric nitrate as precursors. X-ray diffraction, UV-Vis spectroscopy, Fourier transform infrared spectra, scanning electron microscopy, photoluminescence, X-ray photoelectron spectroscopy and photocurrent measurements were used to characterize the prepared catalysts. The results indicated that Fe³⁺ was inserted at an interstitial position of g-C₃N₄ by coordinating to N atoms, which exist in a form completely different from those in the Fe₂O₃/g-C₃N₄ composite. The different form of Fe species caused the difference in optical properties, energy band structure and electrons–holes separation rate. The activities of Fe³⁺-doped g-C₃N₄ and Fe₂O₃/g-C₃N₄ composite catalysts were tested by the photocatalytic degradation of Rhodamine B (RhB) under visible light. The rate constant of Fe³⁺-doped g-C₃N₄ was 2.5 and 1.8 times higher than that of the g-C₃N₄ and Fe₂O₃/g-C₃N₄ composite. The possible mechanism is proposed.