Boehmite AlO(OH) sol was prepared by sol-gel method from aluminum iso-propoxide under an acidic condition and this sol was coated on an alumina support. After it was dried for 2 days at room temperature and calcined at 500℃ for one hour. The membrane had homogeneous ultra fine pores, about 3.0nm, with a narrow pore size distribution and the thickness was in the range of 0.5-8.2㎛, depending upon the number of coatings. Gas permeation characteristics of the membrane with 7.5㎛ thickness were investigated for four gases He, N₂, H₂ and CO₂. Knudsen diffusion was the dominant transport mechanism for these gases. But CO₂ and N₂ showed much higher permeabilities than the values predicted by Knudsen diffusion due to the effect of surface diffusion. The adsorption energy of CO₂ obtained from adsorption iso-therms was 22.40kJ/mol and the calculated surface diffusion coefficients of CO₂ and N₂ turned out to be 10.66×10^-9m²/sec and 10.84×10^-9m²/sec at 290K, respectively. The additional flows by surface diffusion of CO₂ and N₂ was about 43% and 22% of the total flows, respectively. The separation factors of CO₂-H₂ and CO₂-N₂ mixtures approached the theoretical values predicted by Knudsen diffusion with increasing temperature.