The mechanism of reducing fouling by means of ozone was investigated using wastewater from a wastewater treatment facility. Effects such as particle size increase from particle destabilization, organic degradation from high MW to low MW, and bio-fouling reduction from microbiological disinfection were studied. By means of ozone treatment, the permeate flux decline by fouling was improved, E. coli was disinfected completely, the thickness of the foulant layer was reduced by about 50%, and most of high molecular weight (MW) was degraded to low MW, respectively. However, the concentration of TOC in permeates increased by about 70%, and the amount of small particle size distribution (less than 5 mu m) was increased. From these results, the major membrane fouling reduction effect resulted from the degradation of organic compounds, from high MW to low MW, and microbiological disinfection. However, with respect to permeate quality, ozone treatment showed no significantly beneficial effects. The SEM images of the membrane cross-section revealed that the thickness of the foulant layer was about 1.4 mu m with ozone treatment. Without ozone treatment the cross-section image was about 3.2 mu m. This could be due to the fact that foulants degraded from high MW material to low MW and penetrated into the permeate. The increase of TOC concentration was another result of the degradation reaction.