The effect of ozonation on permeate flux was studied by using a polysulfone ultrafiltration membrane. The filtration was carried out by permeating chemical wastewater treated by combined ozone and membrane filtration methods under 3 kg(f)/cm(2) until steady-state flux was obtained. Then, the ozone of concentration range between 10 and 45 mg/lxmin was ozonated in water for reducing the fouling on the UF membrane. It was found that the ozonation made the permeate flux enhance by 10%, and oxidation by ozone and hydrogen peroxide was more effective. Evidence was presented that TMP decreased in more ozone-concentrated water and it was found that the ozone-mediated membrane would have a limited role to prevent membrane fouling rather than to eliminate it fully.