Nitrobenzene (NB) was selected as the model pollutant in water and the efficiency and mechanism of degradation of NB in aqueous solution by O3/H2O2 were investigated. The effects of pH, H2O2 dose and the inhibitor or accelerant of .OH on the removal rate of NB were studied. H2O2 could obviously improve the ozonation decay rate of NB when the pH value of the solution was below 7. The removal rate of NB was enhanced remarkably while H2O2 dose was increased from 1.0 m g/L. to 4.0 mg/L. However, as H2O2 dose increased from 4.0 mg/L to 20 mg/L, the removal efficiency of NB decreased. Different quantities of H2O2 were yielded in different reaction phases of single ozonation system. Both systems of single ozonation and H2O2-catalysed ozonation could not reduce TOC observably. During the NB degradation process, organonitrogen was almost completely converted to nitrate and the pH value of the solution reduced significantly. Results of LC-MS and GC-MS analysis showed that the main intermediate products were phenolic compounds and carbonyl compounds. A possible reaction pathway of the catalytic ozonation of NB was also proposed. It was found that the catalytic ozonation of NB could be divided into two steps. First, hydroxyl radical attacked phenyl ring to form phenolic compounds, then the ring was opened, forming into various aliphatic compounds or being mineralized to inorganic compounds.