Zeolite NaA membranes were prepared hydrothermally by secondary crystallization process at different temperatures (55°C–75°C) on porous α-alumina-based support tubes (inner side) precoated with poly(ethyleneimine) (PEI) buffer layer and NaA seed particles. The NaA seed crystals synthesized at 65°C/2 h in the size range 100–200 nm having BET surface area of 71.57 m2g−1 were used for secondary crystallization of the membranes. The secondary crystallization at 65°C for (4 + 4) h (double-stage) showed highly dense NaA grains in the microstructure of the membrane with a thickness of 5 μm. It rendered the permeance values of 50.6 × 10−8, 2.47 × 10−8, and 0.55 × 10−8 molm−2s−1Pa−1 for H2, N2, and CO2, respectively, with their permselectivity of 20.48 (H2/N2), 92 (H2/CO2), and 4.49 (N2/CO2). A tentative mechanism was illustrated for the interaction of PEI with the support substrate and NaA seed crystals.