Tannic acid, being polyhydroxyl, is a macromolecule natural organic matter. The efficiency and mechanism of degradation of tannic acid by O3 and O3/H2O2 was studied. The results showed that the addition of H2O2 had no obvious improvement on the ozonation efficiency of tannin. It was observed that the addition of H2O2 decreased the removal efficiency of TOC by 10%. The relative intensity of ∙OH generated in the O3‐H2O2 system was detected by electronic spin resonance (ESR) and it was found that tannin consumed the ∙OH radicals, which may be associated with the polyhydroxyl structure of tannin. Derived with PFBBr and analysed by GC-ECD, the by-products of tannin ozonation were found to be aldehydes such as acetaldehyde, glyoxal and methyl glyoxal. Analysis with an ion chromatogram showed that the final products of tannin were oxalic acid, ketomalonic acid and oxalic acid, which consisted of 35% of TOC in both systems.