A new description of the hydrogen permeation through metal membrane has been proposed according to the diffusion equation based on the hydrogen chemical potential. The diffusion of hydrogen atom through metal membrane is found to be linked with the shape of the pressure-composition-isotherm (PCT curve) of the material. Thus, the analysis of the PCT curves is very important to understand the hydrogen permeability of metal membranes. The validity of the new diffusion equation has been verified by a series of hydrogen permeation test with a pure niobium membrane. There is a linear relationship between the hydrogen flux and the PCT factor, f(PCT), indicating that the hydrogen flux obeys the new diffusion equation. This new description of hydrogen permeability is very useful for alloy design and setting the permeation conditions to get high hydrogen flux through metal membrane. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.