The formation and growth mechanism of the wet membrane formed in dip-coating process was systematically studied on the consideration of the capillary colloid filtration and the film coating processes as well as an applied pressure. An expression to describe the growth rate of the wet membrane was derived. It can be denoted that the thickness of the wet membrane increases monotonously with the dipping time but a transition would occur due to the saturation of the capillaries in the substrate and the decrease of the capillary suction force. It is also shown that different mechanism has different effect on the thickness of the wet membrane: the wet membrane formed by film coating may dominate while the dipping time is short and the suspension viscosity is high; but for the suspensions with low viscosity, the capillary colloid filtration contributes more to the wet membrane growth, especially for longer dipping time. As long as the wet membrane exhibits a similar structure during the dip-coating process, the wet membrane thickness can be expressed as a function of the parameters, DeltaP(p), eta, t, etc. And the expression derived in this work has been verified by the experimental data and proved to be valuable for the control of the membrane formation. (C) 2004 Elsevier B.V. All rights reserved.