Analytical results are presented for flow through a porous membrane in the range of pore size which is of comparable magnitude as the mean free path of the fluid molecules. In the present work, we consider a transfer mechanism of combined Knudsen diffusion, slip flow, and viscous capillary flow through a porous flat membrane governed by Brinkman’s equation. The theory shows that as the total pressure of the system increases the Knudsen dif-fusion and slip flow become negligibly small, and that the dominant viscous flow reduces to a Poiseuillian in the limit of large permeability. In addition, as a model of counter-current(or cocurrent)separator module, flow behaviors through a porous hollow-fiber membrane are also analyzed to determine the relationship between the pressure varia-tion and the permeation velocity through the porous membrane.