Novel cross-linked polymers of intrinsic microporosity (PIM-1) membranes have been prepared by post-modification of PIM-1 at the elevated temperature for a period of 0.5 to 2 d. The occurrence of cross-linking reaction has been verified by TGA, XPS and Gel content analyses. TGA analysis indicates an increase in thermal stability of membranes after the thermal cross-linking treatment. There is also obvious drop in the maximum decomposition rate comparing to the original PIM-1when membranes are thermally treated for an extended period of time. XPS results suggest that the nitrile-containing PIM-1 membranes undergo a latent cross-linking reaction, and form stable bulky triazine rings at 300°C. The extent of cross-linking degree increases with the thermal soaking time. As a result, the thermally cross-linked PIM-1 membranes exhibit superior gas separation performance surpassing the most recent upper bounds for all the important gas pairs, such as H2/N2, CO2/CH4 and CO2/N2. PIM-1 thermally treated at 300°C for 2 d has the CO2 permeability of 4000 barrer with CO2/CH4 and CO2/N2 ideal selectivity of 54.8 and 41.7, respectively. These promising results were reconfirmed by the binary gas tests showing similar gas-pair selectivity with about 27-42 % drop in CO2 permeability due to the competitive sorption in the penetrant gases.