Different　pretreatments,　including　coagulation,　adsorption,　and　biofiltration,　were　used　to　identify　the　useful　pretreatment　for　the　ultrafiltration　of　polluted　raw　water.　The　following　prefiltration　investigation　was　conducted　to　identify　foulants　that　were　responsible　for　membrane　fouling.　Organic　removal　and　particle　size　distribution　of　different　pretreated　samples　were　measured.　The　results　indicated　that　the　concentration　of　organic　matter　was　not　the　critical　factor　that　determined　the　extent　of　flux　decline.　The　turbidity　relevant　materials,　such　as　colloids　and　particles,　were　supposed　to　be　important　fouling-caused　substances,　and　the　particle　size　distribution　also　had　a　significant　influence　on　flux　decline.　Moreover,　the　fouling　resistance　of　different　prefiltrated　samples　showed　that　foulants　sized　over　the　membrane　pore　size　caused　the　main　part　of　the　membrane　fouling　in　raw　water.　Component　analysis　demonstrated　that　protein-like　substances　played　a　crucial　role　in　membrane　fouling;　however,　humic-like　substances　led　to　a　weak　membrane　fouling.