The effects of dynamic membrane formed by fine particles on the filtration rate and the rejection of macromolecules in cross-flow microfiltration are studied. Experiments were carried out using the binary suspensions prepared by polymethyl methacrylate (PMMA) particles and the Dextran macromolecules. The filtration rates, the rejections of Dextran and the cake properties under various operating conditions, such as the cross-flow velocity and the filtration pressure, are measured and discussed. The filtration rate increases with increases of the cross-flow velocity and the filtration pressure but decreases when the molecular weight of Dextran is increased. On the other hand, the rejection of Dextran increases with the increase of filtration pressure but decreases with the increase of cross-flow velocity. The standard capture equation for deep-bed filtration is adopted to relate the rejection of Dextran and the properties of dynamic membrane. The results show that the mass of the dynamic membrane is the most important factor on the rejection of Dextran. Using the experimental data obtained under various cross-flow velocities, the rejection of Dextran can be regressed as a single function of cake mass. It can be concluded that the rejection of macromolecules increases due to the presence of fine particles, and that the magnitude of the rejection increases with increasing the mass of cake.