g-C3N4 attracts huge research mainly being used as a photocatalyst, but its Fenton-like catalytic performance has been overlooked. In this paper, the dark Fenton-like catalytic performance of g-C3N4 was evaluated by degrading rhodamine B over a wide pH range. The results showed that the g-C3N4, which was synthesized by conventional urea pyrolysis without any modification, was an efficient metal-free heterogeneous Fenton-like catalyst. The highest activity occurred under a weakly alkaline condition of about pH 10. The experiment of catalyst recycling indicated that g-C3N4 had long-term stability. The reactive oxidizing species of HO·, generated by the g-C3N4 activating H2O2, was identified by ESR and further supported by a scavenging experiment of HO· using isopropanol as the scavenger. The HNO3 oxidation of g-C3N4 resulted in catalytic deactivation, implying the catalytic activity originated from the surface reduced groups of g-C3N4. The structure of synthesized g-C3N4 before and after the HNO3 oxidation was characterized by XRD, FTIR, XPS, and a possible catalytic mechanism was proposed.