We have developed functional poly(1,5-naphthalene-2,2'-bis(3,4-phthalic)hexafluoropropane diimide (6FDA-NDA)/polyethersulfone (PES) dual layer hollow fiber membranes and studied the H-2/CO2 separation performance with and without 1,3-diaminopropane (FDA) modification. The effects of air gap on the membrane gas transport properties are investigated and it is observed that the optimal air gap for maximizing the ideal permselectivity is strongly dependent on the kinetic diameters of gas pair molecules. A higher air gap diminishes the population of Knudsen pores in the apparently dense outer skin and induces greater elongational stresses on polymer chains. The latter enhances polymer chain alignment and packing which possibly results in the shift and sharpening of the free volume distribution. The diamine modification of 6FDA-NDA/PES dual layer hollow fiber membranes effectively enhances the H-2/CO2 permselectivity and suppresses the CO2-induced plasticization. At higher temperatures, the H-2 permeance of the PDA-modified fibers increases with negligible decrease in the H-2/CO2 permselectivity.