We have developed an extremely simple room temperature chemical cross-linking technology for the modification of polyimide films for gas separations of He/N2 and O2/N2. Using 6FDA-durene as an example, chemical modification is performed by immersing the dense 6FDA-durene films in a p-xylenediamine methanol solution for a certain period of time followed by washing with fresh methanol and drying at ambient temperature. The chemical structure changes during the cross-linking process were monitored by FTIR, which indicated that imide groups were turned to amide groups during the modification process. TGA analyses showed cross-linked polyimides were thermally stable for gas separation applications. Gas permeation properties of modified polyimides to He, O2, N2 and CO2 were measured at 35°C and 10 atm. It is found that the gas permeability decreased significantly in the order of CO2>N2>O2>He with an increase in the degree of cross-linking, which were mainly attributed to the significant decreases in diffusion coefficients. The permselectivities of He/N2 and O2/N2 increased from 10 to 86 and from 4.1 to 5.9, respectively, but CO2/N2 decreased from 12 to 5.4, which suggest this cross-linking approach is most useful for the application of He/N2 and O2/N2 separations.