This study deals with the ion beam irradiation effect on gas permeation properties of polyimide films. 2 MeV alpha, 500 keV, and 170 keV N+ ions were used for modifying the membranes. It was found that there are two different effects according to the implanatation dose. In the case of small-dose irradiation, ion implantation causes a raise of permeability both for CH4 and H2. When the implantation dose reaches a more important level, the implanted membranes have at the same time high permselectivity for H2/CH4 and high permeability for H2. The relationships between the permeation properties and microstructure of the films are also discussed.