Simulation studies in pure water were conducted to investigate the effect of nano-metal oxides on bromate (BrO 3 − ) formation as catalysts and the catalytic mechanism. Results indicated that compared to ozonation alone, both nano-SnO 2 and nano-TiO 2 could inhibit the formation of bromate during ozonation process. The inhibition efficiency of BrO 3 − formation by nano-TiO 2 enhanced with the increasing of ozone dosage and the decreasing of nano-TiO 2 dosage, Br − concentrations and the pH value. Possible BrO 3 − minimization mechanism was that nano-TiO 2 accelerated the decomposition of the dissolved O 3 into OH radicals, which rapidly generated H 2 O 2 , and reduced HOBr/OBr − to Br − .