The gas separation properties of vacuum ultra violet (VUV) irradiated polyimide membranes and surface-crosslinking polyimide membranes after VUV irradiation were investigated. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and the X-ray photoelectron spectroscopy (XPS) measurements of the membrane showed the increase in the number of hydroxyl groups after VUV irradiation and the formation of hydrogen bonds from the hydroxyl groups. The gas permeability of the surface-modified polyimide membrane decreased with increasing gas molecular size. Moreover, the permselectivity to H-2/CO2, H-2/N-2, and H-2/CH4 of the polyimide membranes irradiated with VUV and treated with 3-aminopropyltriethoxysilane (APTES) increased from 0.74 to 17, 13 to 230, and 14 to 600, respectively. Therefore, we successfully established a network structure by using hydroxyl groups formed by VUV irradiation and improved the hydrogen separation performance of the polyimide membranes.