The effects of membrane pore size and operating conditions on the particle fouling in "dead-end" microfiltration are studied by use of membrane blocking models. Two track-etched membranes (Isopore (R) membrane) with the mean pore diameters of 0.2 and 0.4 mu m, respectively, are used as the filter media to filter 0.15 mu m polymethyl methacrylate (PMMA) particles. A blocking chart is established for relating the operating conditions and the particle fouling for the testing materials. The major factors affecting the blocking index, particle accumulation and filtration flux, are discussed thoroughly. The blocking index varies gradually at the initial period of filtration, keeps at ca. 0.5 for a while and then suddenly drops to zero at a critical condition. The blocking index for 0.4 mu m membrane is always larger than that for 0.2 mu m membrane under the same filtration pressure and filtration flux due to more severe membrane blocking. The critical point at which the filtration transforms from membrane blocking to cake filtration occurs at a lower filtration flux or a lower particle accumulation for the membrane with larger pore size or under lower filtration pressure. The normalized resistance coefficient can be correlated to a unique function of blocking index under various operating conditions and for different membranes.