Nanosized TiO2 catalyzed ozonation of trace nitrobenzene in water was carried out in an agitated slurry semi-batch. It demonstrated that removal rate of nitrobenzene with catalytic ozonation increased by 44% than ozonation alone. Effect of heat treatments of TiO2, catalyst mass, the initial concentration of ozone and nitrobenzene, and pH were investigated respectively. TiO2 calcined at 550 degrees C showed the best catalytic activity. In the presence of TiO2 calcined at 550 degrees C, about 56.57% of nitrobenzene in the solution could be ozonated. Both ozone dosage and initial concentration of nitrobenzene exerted a positive influence on the catalytic ozonation. But the increase of the catalyst mass did not yield any significant increase of the nitrobenzene removal. The efficiency of catalytic ozonation was special lower at acid pH than basic pH. It was also observed that both ozonation and catalytic ozonation were significantly influenced by tert-butyl alcohol, which testified that nanosized TiO2 catalyzed ozonation of nitrobenzene follows a radicaltype mechanism.