In order to know the possibility of CO2 separation by surface diffusion with ceramic membrane at high temperature, permeability of CO2 was estimated from the heat of adsorption of CO2 on gamma-alumina and MgO-modified gamma-alumina. It is revealed that the too high heat of adsorption retards the mobility of CO2. Ordinary gamma-alumina was preferable for the CO2 separation. Estimated permeability of CO2 by surface diffusion was larger than that by molecular sieve mechanism.