A polyimide membrane, 6FDA-TMPD, was prepared from 5,5＇-[2,2,2-trifluoro-1-(trifluoromethyl)ethylidene]bis-1,3-isobenzofurandione (6FDA), and 2,4,6-trimethyl-1,3-phenylenediamine (TMPD). The effect of ultraviolet irradiation (UV irradiation) using a high-pressure mercury lamp on the gas transport properties of the 6FDA-TMPD membrane and membranes containing a photosensitizer was explored. Comparison between UV treatment and thermal treatment was also examined. The gas selectivities in all membranes were increased due to the irradiation. Especially in the H-2/N-2 and H-2/CH4 separations, the selectivities effectively increased without a significant reduction in the H-2 permeability. These increases were considerable in the membrane having no photosensitizer which was expected to be an initiator for the cross-linking reaction. There was little effect of cross-linking, which was formed by the irradiation, on the gas permeation properties. Photosensitizers prevented drastic change in the membrane morphologies due to the irradiation.