Ultrafiltration　performance　of　wastewater　effluent　organic　matter　(EfOM)　and　background　natural　organic　matter　(NOM)　under　different　molecular　weight　cutoff　(MWCO)　membranes　was　investigated.　Permeates　of　EfOM　and　NOM　from　ultrafiltration　were　systematically　compared　by　using　fluorescence　spectroscopy　and　gel　filtration　chromatography.　The　results　revealed　that　anthropogenic　polysaccharide-　and　protein-like　substances　including　polysaccharides,　proteins,　and　colloids　in　EfOM　were　mainly　removed　by　ultrafiltration　with　the　100　kDa　membrane,　which　accounts　for　a　large　proportion　of　the　total　DOC　in　EfOM.　With　the　decreasing　of　the　membrane＇s　MWCO,　the　humic　substances　in　NOM　were　more　easily　removed　compared　with　that　in　EfOM.　The　polysaccharides　associated　with　the　colloidal　fraction,　as　well　as　the　humic　substances　were　speculated　as　the　main　potential　foulants　for　UF　membranes　processing　wastewater　EfOM.　As　for　NOM,　humic　substances　were　regarded　as　the　main　potential　foulants.　The　fouling　mechanism　(cake　filtrations　or　pore　blocking)　of　EfOM　or　NOM　in　the　ultrafiltration　process　was　related　with　the　MWCO　of　the　membrane　and　the　molecular　size　of　the　main　foulants.　(C)　2014　Elsevier　B.V.　All　rights　reserved.