A concept for alloy design of hydrogen permeable membrane with high hydrogen permeability and long-term durability has been proposed in view of the PCT factor, fPCT, and the ductile-to-brittle transition hydrogen concentration, DBTC. As an example, V–10mol%Fe alloy has been designed for low operative temperature, which exhibits excellent and stable hydrogen permeability for at least 1000 hours at 573 K without brittle fracture.In addition, the alloying effects of iron on the hydriding property and the hydrogen diffusivity have been investigated quantitatively in order to establish a way to design optimal composition of V-Fe based hydrogen permeable alloy under any given conditions. It is found that the addition of iron into vanadium increases linearly the partial molar enthalpy change, ΔH0.2, of hydrogen for hydrogen dissolution, but scarcely affects on the partial molar entropy change, ΔS0.2. It is also found that both the activation energy, E and the pre-exponential factor, B0, of the mobility for hydrogen diffusion decrease linearly with increasing the mole fraction of iron, meaning that the addition of iron enhances the hydrogen diffusivity at low temperature below about 700 K. The evaluation in view of the four parameters, ΔH0.2, ΔS0.2, E and B0, is useful for deep understanding of the property of hydrogen permeable metal membrane. Following the concept for alloy design in view of these four parameters, optimal alloy composition can be designed under any given conditions. The hydrogen permeability of the designed alloy under the condition can also be estimated quantitatively.