In this study, the gas permeance and selectivity of the asymmetric polyimide membrane irradiated by He ions were measured using a high vacuum apparatus with a Baratron absolute pressure gauge at 76 cmHg and 35 degreesC. The asymmetric polyimide membranes were prepared from a dry-wet phase inversion process, and the surface skin layer on the membrane was irradiated by He ions in the fluence range from 1 x 10(12) to 3 x 10(15) (He+/cm(2)) at 50 keV. We demonstrated that the gas selectivity of the He+-irradiated membrane increased with an increase in the ion fluence and that the depth profile of the energy loss for the irradiating ions in the skin layer had a significant influence on the gas permeability and selectivity of the asymmetric membrane.