Permeate flux decline in a microfiltration was analyzed by measuring the permeability of Al2O3 colloidal solution through polyethylene capillary membranes. The flux decline was due to the growth of cake layer on the membrane surface and to the pore blocking by particles. When a steady state is approached, the permeate flux was ultimately controlled by the cake filtration model. For the membrane with 0.34 μm pore size, the decline was slower than that with 0.24 μm, and the dominant fouling model was seemed to be cake filtration following pore plugging at the initial stage. Permeate flux for the colloidal suspension of 0.6 μm Al2O3 particles was less than that of the 1.0 μm Al2O3 particles, and the approach to the steady state was fast. At high transmembrane pressure, the flux decline was fast attributing to the formation of denser packed cake layer and pore blocking. With increasing the feed flow rate from 12 L/h to 24 L/h, the permeate flux increased 17% for the colloidal suspension of 0.6 μm particles. The cake filtration model was dominant at the initial period of filtration, however, the filtration followed pore-blocking model as time progressed. The initial flux decline was greater with the mixture of 0.6 μm and 1.0 μm Al2O3 particles than with 1.0 μm particles alone in the solution.