A poly [2, 2'-(p-oxydiphenylene)-5, 5'-bibenzimidazole] (OPBI) was employed to pyrolyze under an inert Ar atmosphere to produce alumina-supported carbon molecular sieve membranes (CMSMs) for CO2/CH4 gas separation. Pyrolysis temperature was also varied from 550 degrees C to 750 degrees C to tailor the carbon microstructure and then optimize the separation performances of CMSMs. A structure-performance relationship was established that high pyrolysis temperature can promote the generation of a compact and ordered graphite carbon structure and further give rise to a remarkable combinations of separation factor and gas permeability of CMSMs. Much effort has been made to reduce the thickness of CMSMs via lowering the concentration of polymer precursors or increasing the dip-coating withdraw speed. The optimal thinner CMSMs separately prepared from diluted OPBI polymer solution and increased dip-coating withdraw speed revealed a remarkable improvement of CO2 permeance, transcending the Robeson's upper bound of polymer membranes and of great potential in natural gas purifications.