The pretreatment of raw seawater is necessary to minimize the organic fouling of seawater reverse osmosis (SWRO) membranes. To predict the membrane fouling of the pretreated seawater, the modified fouling index (MFI) with ultrafiltration (UF) was investigated in terms of molecular weight distribution (MWD) and membrane characterization. The study was conducted with seawater drawn from Collioure, France. The concentration of total dissolved solid was 32760 mg/L. The molecular weight (MW) of the initial seawater organic matter (SWOM) ranged from about 14160 Da to 280 Da. FeCl 3 flocculation removed the majority of SWOM, while PAC adsorption could not remove the lowest MW fraction of organic matter ( 1110, 750 and 280 Da) . The UF membranes with 30 kDa and 100 kDa molecular weight cut-off removed the majority of of organic matter corresponding to the peaks 14160 Da and 6560 Da in Molecular Weight Distribution.The MFI values obtained when using UF membranes of 30 kDa and 100 kDa with MF pretreatment were 19700 s/L 2 and 31000 s/L 2 , respectively. The MFI values after pretreatments of FeCl 3 flocculation and PAC adsorption significantly decreased to 6900 s/L 2 and 6700 for 30 kDa UF and to 2300 s/L 2 and 2500 s/L 2 for 100 kDa UF, respectively. Some relation does exist for both membranes between the MFI-UF values and (S pb ) values obtained during the first peiod of filtration (pore blocking). The pore blocking zone significantly decreased after flocculation and adsorption pretreatment. This suggests that the pore blocking can be used as an indicator to predict membrane propensity. The detailed membrane characterization on the clean and fouled membrane surface after MFI-UF experiments was made in terms of contact angle, zeta potential, functional group and microscopy.