The mechanism of cake formation is studied in 2–D cross-flow filtrations. By analyzing the forces exerted on a depositing particle, the value of the critical friction angle, βc, which leads the particle to be stuck on the contact position, is determined. Not only the probability of particle deposition but also the size distribution of deposited particles can be calculated from the theoretical value of βc. Numerical programs are constructed to simulate the packing structure and size distribution of particles in a formed cake and the path of cake compression. The local cake properties in the cake, such as local porosity and local specific filtration resistance, are also estimated by applying the design program and the data of filtration volume vs. time. The predicted values of average cake porosity and average specific filtration resistance agree very well with the experimental data.