Owing to their unique structure and specific function, layered-structure nanocarbons, including graphene oxide (GO), reduced GO (rGO), and graphitic carbon nitride (g-C3N4), show potential activity as metal-free catalysts in heterogeneous catalytic ozonation for water purification. In this study, layered-structure nanocarbons GO, rGO, and g-C3N4 were applied as metal-free catalysts in catalytic ozonation to investigate the mechanism and driving force of reaction oxygen species formation. Although GO showed excellent catalytic activity, its instability was a decisive factor in its catalytic ozonation mechanism, due to the layered structure and chemical properties destroyed by ozone and formed free radicals. Meanwhile, the structure of rGO was preserved in catalytic ozonation, enabling a stable and kinetics-enhanced catalytic reaction. Interestingly, g-C3N4 showed activity in catalytic ozonation for refractory organic compound degradation and bromate elimination for the first time. X-ray photoelectron spectroscopy and in-situ electron paramagnetic resonance spectra showed that surface oxygen-containing functional groups (such as -OH, -C=O, and -COOH) in layered-structure nanocarbons were sequentially reduced either by direct oxidation or by promoting ozone decomposition to form free radicals and H2O2. The original inherent delocalized electrons generated by electron-rich nitrogen vacancies in g-C3N4 promoted ozone decomposition to generate hydroxyl radicals. Through systematic study, new insights into the catalytic ozonation processes of GO, rGO, and g-C3N4 were evaluated and explained. This work presents an extensive investigation into catalytic ozonation using layered-structured nanocarbons, which will be important for the design of nanocarbons-based catalysts and improve understanding of the principles of heterogeneous catalytic ozonation.