> Secondary growth technique was successfully applied for the synthesis of nanostructure sodalite membranes with vacuum seeding on tubular α-Al2O3 supports. In the seeding process, a thin, uniform and continuous seeding layer was closely attached to the support external surface by the pressure difference between the two sides of the support wall. The final nanostructure sodalite top-layers were synthesized on the seeded support into a stainless steel autoclave with a Teflon holder. The effect of seeding time on the microstructure of the synthesized sodalite top-layers was investigated at four different levels 60, 120, 180 and 240 s. The synthesized membranes were characterized by XRD, SEM and mercury porosimetry. The obtained results showed that sodalite zeolite was synthesized on the membranes top-layers with uniform surfaces. But, the top-layers thickness increases by increasing the seeding time and tends to reach a plateau. Further increasing the seeding time causes dense top-layers to form. Also, the performance of the manufactured sodalite membranes was evaluated by permeations of single gases (H2 and N2) under different pressure differences at a temperature of 283K. It is found that the membrane permeance shows a maximum value at seeding time 180 s with a constant permselectivity (H2/N2) of about 2.5. The permeation results were confirmed by SEM micrographs which showed a thick and low-dense top-layer in the membranes manufactured with the seeding time of 180 s.