The microporous alumina membrane with asymmetrical structure, having upper layer with pore diameter of 10 nm under and lower layer with pore diameter of 36 nm, was prepared by anodic oxidation using DC power supply of constant current mode in an aqueous H2SO4 solution as a electrolyte. The aluminum plate was pretreated with thermal oxidation, chemical polishing and electrochemical polishing before anodic oxidation. The membranes were prepared by controlling the current density such as a very low current density for upper layer and a high current density for lower layer of membrane. By controlling the cumulative charge density, the thickness of upper layer of membranes was about 6 mum and the total thickness of membranes was about 80-90 μm. We found from gas permeation experiments with the membranes prepared by above method that the mechanism of gas permeation of the all membranes prepared under each condition complied with model of the Knudsen diffusion.