In this study, the effect of coagulation pretreatment on improvement of UF membrane filtration flux was investigated. The experiment using four water samples having different hydrophobic and hydrophilic compound was focused on the effect of coagulation on removing hydrophobic and hydrophilic fraction of organics and resulting enhanced flux. In the filtration of raw water, the organics removal for the four water samples were in the ranges of 12% and 20%, in which hydrophobic fraction accounting for over 50%, suggesting that membrane prefers to remove hydrophobic fraction. In the addition of 25 mg/L coagulant, the organics removals for the four water samples were in the ranges of 12% and 28%. As coagulant dosage increased to 100 mg/L, removals increased to 25% and 38% accordingly, in which hydrophobic fraction accounting for over 50%, suggesting that coagulation prefers to remove hydrophobic fraction. Coagulation pretreatment could enhance flux of all of water samples studied. The analyses for each organic compound show that after coagulation pretreatment membrane reject hydrophilic fraction mainly due to removal of hydrophobic fraction effectively by coagulation. It can be concluded that the flux decline of UF membrane was mainly caused by hydrophobic compound and hydrophilic compound had less effect on flux.