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UF membranes are widely used in water and wastewater treatment for the removal of particles in place of rapid sand filtration. One thing unclear is the effect of membrane molecular weight cutoff (MWCO) on particle deposit and resistance build-up on the membrane. In this paper, the authors investigated the performance of polyethersulfone (PES) membranes of three MWCO levels as 10k, 50k, and 100k Da for the filtration of kaolinite clay suspensions. A particle dispersion analyzer (PDA) was introduced to the study for monitoring the variation of particle concentration in the concentrate. Using the PDA online monitoring data and conducting mass balance analysis of particles in the UF filtration system, the deposit weight on the membrane surface was calculated. Analysis was further conducted on the relationship between the unit surface deposit weight and resistance build-up on the membranes of different MWCOs according to Darcy's law and using flux data. As a result, a PES membrane of smaller MWCO was found to accumulate larger quantity of deposit per unit permeate volume than that of larger MWCOs. Furthermore, with the same weight of membrane deposit, higher hydraulic resistance was built up on smaller MWCO membrane than the larger ones. The PDA online monitoring technique provided a new approach for the study of membrane fouling by suspended particles.