The composite membranes of gamma-Al2O3 were fabricated by sol-gel coating with boehmite (AlOOH) sol, The thickness of the membranes were in the range of 2-5 mu m depending on the number of coatings and the concentration of the sol. The permeabilities of He, N-2, and CO2 were measured at temperatures of 298-673 K and pressures of 101.3-405.2 KPa. The permeability through gamma-Al2O3 membrane was constant with the pressure difference and decreased with the square root of temperature. The main mechanism of gas transport through gamma-Al2O3 membrane was thus proved as Knudsen diffusion. CaO was impregnated on the gamma-Al2O3 membrane to improve separation factor by introducing interactions between CO2 gas molecules and the pore wall, but high separation factor was not obtained. Silica modified gamma-Al2O3 membrane was also prepared to increase the separation factor. The separation factor of CO2 to N-2 was increased to 1.4 at 298 K though it decreased with increasing temperature. The separation experiment of a CO2/N-2 mixture was also performed. The separation factor was 1.72 at 298 K and 1.5 at 673 K, which were much higher than that of the Knudsen diffusion mechanism, 0.8.