Abstract: In this paper,a gas permeability measurement technique based on the modified transition flow conductance theory is proposed,by which the characteristic size of nano-porous media can be characterized nondestructively.The conductance of the gas through the porous media in the transition state was measured in real time by the dynamic differential pressure transient test system.And the relationship between the gas conductance and the mean free path of the gas was obtained,from which the average diameter of the porous media material was derived.Two groups of porous anodic alumina(AAO)with different areas were tested.The test results are consistent with the SEM image analysis results,and the relative error is within 8%.In this method,the porosity of the porous media is not required before the test,and it has the advantages of non-destruction,accuracy,and large-area-measurement,which extends the test range of gas permeability measurement technology.