Abstract: α-Al₂O₃ membrane supported on α-Al₂O₃ porous substrate was formed by dip-coating and the formation process of the top layer was investigated based on experiments and theories of capillary filtration and film-coating. A model was developed to interpret quantitatively the formation process of the top layers on the porous substrate and all variables affecting top layer thickness were considered in the model.The results of theoretical analysis showed that the effects of six variables,namely the mass concentration of ceramic powder(W_s), the viscosity of suspension (η), the diameter of ceramic powder(r), the pure water permeability of support(Q),dipping time(t) and the withdrawal speed of the support(U),on the thickness of the top layer were highly significant. In order to validate the model developed,ZrO₂ membranes were prepared by dip-coating using the slurry with different mass concentrations of ZrO₂ powder. The experimental data of the thickness of the as-prepared ZrO₂ membranes showed good agreement with the model.