Pre-coagulation and then flocculation with/without sedimentation before ultrafiltration (CSUF/CUF) was compared to explore the process for treating synthetic humic rich water and controlling membrane fouling. The sedimentation process caused larger flocs to be removed and zeta potential of the remaining flocs to be more negative. Also the fractal dimension of flocs became smaller after sedimentation process. All the data seemed to suggest that sedimentation process can improve the membrane fouling. On the contrary, the development of trans-membrane pressure (TMP) in CSUF surprisingly increased higher than that in CUF. The scanning electron microscope (SEM) images of cake layer on the surface of membrane showed that the size of nano-scale primary particles formed in coagulation process was heterogeneous. It also showed that the flocs formed by larger nano-scale primary particles were removed by sedimentation process, and the smaller nano-scale primary particles in CSUF maybe the crucial reason causing higher development of TMP. The density of cake layer on the CUE membrane surface with larger nanoscale particles was much lower than that by CSUF. Therefore, the characteristic of flocs, especially the size of nano-scale primary particles, may be the main factor inducing higher increase of TMP in CSUF, which should be explored in the future. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.