Polycrystalline ZIF-8 membranes of about 1 mu m in thickness were successfully grown on alpha-alumina porous support by seeding-free aqueous synthesis. In order to achieve continuous well-intergrown membranes, a pseudo-surface of ZIF-8 was formed on the support by chemical modification. Our membrane preparation in aqueous system is based on the support-surface activation concept to control the surface density of nucleation sites. The grain size of polycrystalline membrane is directly related to a graft density of ligand-analogous surface modifier. Single-component gas permeation properties were investigated using H-2, CO2, N-2, CH4, C3H6, and C3H8. While the ideal permselectivities of H-2 from CO2, N-2, CH4, and C3H6 ever so slightly increased with the graft density, the ideal permselectivity of C3H6/C3H8 increased rapidly and reached 36 with corresponding C3H6 permeance of 8.5 x 10(-8) mol/m(2) s Pa. As a conclusion of the combined characterization with adsorption analysis, it was found that the sieving of C3H6/C3H8 by our seeding-free aqueous-synthesized ZIF-8 membrane is mainly governed by diffusion-driven separation process.