Novel thermally self-cross-linked polymers of intrinsic microporosity (PIM-1) membranes have been prepared by postmodification of PIM-1 at the elevated temperature for a period of 0.5-2 days. The occurrence of cross-linking reaction has been verified by thermogravimetric analysis (TGA), X-ray photoelectron spectrometer (XPS) and gel content analyses. TGA analyses indicate an increase in thermal stability of membranes after the thermal cross-linking treatment. There is also an obvious drop in the maximum decomposition rate comparing to the original PIM-1when membranes are thermally treated for an extended period of time. Both FTIR and XPS results suggest that the nitrile-containing PIM-1 membranes undergo a latent cross-linking reaction, and form stable bulky triazine rings. The resultant cross-linked polymeric membranes exhibit exceptional gas separation performance that surpasses the most recent upper bound of state-of-the-art polymeric membranes for the important gas separations, such as hydrogen purification, CO2 capture and flue gas separation. In addition, both gas permeability (attributed to the contorted nature, rearrangement and pronounced inefficient packing of PIM polymer chains) and selectivity (attributed to the decrease of chain-to-chain spacing) increase diagonally with the upper bound line when thermal soaking time increases. PIM-1 thermally treated at 300 degrees C for 2 days has the CO2 permeability of 4000 barrer with CO2/CH4 and CO2/N-2 ideal selectivity of 54.8 and 41.7, respectively. The thermally cross-linked PIM-1 membranes will probably provide a promising alternative in industrial energy development.