To　investigate　the　main　factor　that　influenced　membrane　fouling　in　ultrafiltration,　a　simulated　process　of　powdered　activated　carbon　and　ultrafiltration　(PAC/UF)　was　used　to　treat　micro-polluted　raw　water　and　the　sedimentation　effluent.　Membrane　fouling　characteristics　of　two　samples,　effect　of　particle　size　and　dissolved　organic　matter　on　membrane　fouling　were　investigated.　The　results　indicated　that　PAC/UF　process　could　mitigate　the　membrane　fouling　of　the　sedimentation　effluent,　but　it　could　not　alleviate　the　membrane　fouling　of　raw　water.　Organics　in　the　raw　water　were　mostly　hydrophilic　organic-based　small　molecules,　and　it　could　not　mitigate　the　membrane　fouling　of　the　raw　water　although　the　aromaticity　and　the　dissolved　microbiological　products　were　reduced　with　the　adsorption　of　PAC　particles,　which　indicated　that　the　adsorbed　organic　did　not　cause　serious　membrane　fouling　in　the　raw　water.　The　particles　in　raw　water,　with　a　small　particle　size　and　high　number,　was　thought　to　be　the　main　factor　that　leading　to　the　invalid　fouling　control　function　of　PAC/UF　process　on　raw　water.　Particle　number　and　particle　size　distribution　have　a　critical　role　in　membrane　fouling.