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.