Tumor microenvironment-responsive chemodynamic therapy (CDT), an approach based on Fenton/Fenton-like reaction to convert hydrogen peroxide (H2O2) into the highly cytotoxic hydroxyl radical (·OH) in situ to kill cancer cells, represents an important direction for cancer therapy. Different types of nanozymes (nanomaterial-based catalysts that can mimic the activities of natural enzymes) have been developed to mimic peroxidase. This Perspective highlights the latest research progress regarding low-cost and biocompatible carbon-based nanozymes for peroxidase mimics. The effects of structure and surface properties of carbon-based nanozymes on their electronic transfer and peroxidase-like activity are analyzed, including nanospheres, nanotubes, nanosheets, and graphene quantum dots (GQDs) with or without surface functionalization and heteroatom doping. We expand our newly developed carbon nitride (g-C3N4) QD systems to nanozyme application, which are highly efficient in converting the intracellular H2O2 to ·OH species to kill 4T1 cancer cells and demonstrate a great potential for CDT.