The purpose of this study is to understand the effect and mechanism on preventing membrane fouling by coagulation pretreatment in terms of fractional component and molecular weight of NOM. A relatively high molecular weight (MW) of hydrophobic compounds was responsible for rapid decline in ultrafiltration flux. Coagulation could effectively remove hydrophobic organics, resulting in the increase of flux. It was found that lower MW of neutral hydrophilic compounds, which could be poorly removed by coagulation, was responsible for the slow flux decline. The flux in the filtration of coagulated and supernatant water was compared and the results show that lower MW of neutral hydrophilic compounds which remained in the supernatant water after coagulation could be rejected by the membrane, resulting in fouling. It was also found that coagulated flocs could effectively absorb neutral hydrophilic compounds. Therefore, with the coagulated flocs formed on the membrane surface, the flux decline could be improved.