Recently engineered porous materials have been attracted more attentions because of their potential applications in several industrial sectors, such as high efficiency gas separation membrane, catalytic membranes for chemical processing and so on. In spite of their desirable applications, these porous materials have not been realized due to several technical barriers, for example, lack of pore size control. There is a general viewpoint that for conventional gas separation membranes one cannot increase both permeability and permselectivity of the product gas simultaneously. This general viewpoint implies that the conventional methods of gas separation membrane fabrication cannot precisely control the pore size. In this work iodine diffusion property and gas permeation properties of ion beam irradiated polyimides films have been investigated. Small fluence irradiation can raise significantly the iodine penetration in the energy deposition range and permeabilities of the films both for H2 and CH4. After higher fluence irradiation, the implanted layers become stopping layers blocking iodine diffusion into the film and both the permeability for H2 and permselectivity for (H2/CH4) of the film increase hugely. We related the small fluence effects to defects created by irradiation and higher dose effects to the new materials formation of which the pore size is well defined. From these results we can perhaps have a peep at a new way to microporous materials.