Viscous flow, Knudsen diffusion, and surface diffusion models are commonly accepted in diffusion inside porous materials and permeation through porous membranes, including polymeric and inorganic materials. However, the permeation equation through subnano?porous membranes is still under discussion. In this contribution, permeation models of gaseous molecules through porous membranes, including gas translation model (GT model) and uniform potential model, are summarized, and then the modified GT model that was recently proposed, was introduced. In addition, Normalized Knudsen?based Permeance (NKP), which was theoretically derived based on the modified GT model, was introduced to the evaluation of pore sizes of porous membranes with sub?nanometer in size.