The mechanism of cake formation in constant pressure filtrations of submicron particles is studied by analysing the forces exerted on the depositing particles. The Brownian dynamic method is used to trace the loci of slurry particles and to simulate the packing structure of submicron particles in filtrations. The major factors affecting the particle packing, such as frictional drag, van der Waals' force, electrostatic force, and Brownian force, etc., are discussed. The most compact cake is found when the frictional drag and Brownian force are of the same order of magnitude. A numerical program is designed to estimate the variations of local cake properties during a course of constant pressure filtration. The calculated results of average cake porosity and average specific filtration resistance agree very well with the experimental data.