Abstract: The effect of average pore size of carbon membranes for gas separation on the membrane separation characteristics was discussed. It was shown that average pore size of membrane,which determined the mechanisms of mass transfer through the membrane,was the substantial factor of the membrane separation characteristics. As to separation of N₂/O₂ in the transition region of Knudsen diffusion and molecular sieving, two characteristic pore sizes corresponding to the largest separation power and non-selectivity exist.