This paper focused on the major particle-size based foulants responsible for membrane fouling of ultraflltration in simulated surface water polluted by sewage discharge. The experiment was conducted with a sequential filtration process, pre-filtration steps removed specific foulants (1.2 mu m, 0.45 mu m, 100 kDa and 10 kDa) from raw water with different membranes, and followed filtration tests were performed to investigate the fouling caused by remaining foulants. Flux decline and specific fouling resistance of different fractioned substances were compared to identify the particle-size based foulants most responsible for membrane fouling. The normalized flux decline showed that foulants over 1.2 mu m had limited influence on flux decline, but that the 100 kDa-0.45 mu m fraction contributed a significant part. Results of specific fouling resistance revealed that the 100 kDa-0.45 mu m fraction caused higher fouling resistance than other fractions with the same quality of organic matter, and this fraction was confirmed to be most closely related to protein-like substances. Component analysis demonstrated that protein-like substances played a crucial role in membrane fouling, whereas humic-like substances were of minor relevance. Standard blocking and cake filtration were major mechanisms of membrane fouling caused by pre-filtered water, independent of the pore size of fractionation membranes. (C) 2014 Elsevier B.V. All rights reserved.