Gas separation by metal-organic framework (MOF) membranes are still unsatisfactory due partly to unsatisfied separation characteristics caused by the trade-off between selectivity and permeability. Herein, we provide a facile post CO2 annealing method to remarkably improve both the permeance and separation factor of IRMOF-1 membranes for CO2 separation from CO2/H-2 mixture. Post-treatment of the IRMOF-1 membrane by annealing at 100 degrees C under a high pressure CO2 stream deceases H-2 permeance while increasing CO2 permeance, leading to simultaneous enhancement in both CO2/H-2 separation factor from 721 up to 5781 and CO2 permeance from 5.67x10(-7) up to 9.38x10(-7) molm(-2) s(-1) Pa-1 at CO2 molar fraction of 98%, feed pressure of 5 atm and 298 K. The unusual separation behavior is related to the enhanced CO2 adsorption selectivity over H-2 and reduced CO2 affinity of the membrane due to the formation of surface carbonate anions caused by the CO2 treatment. The proposed gas atmosphere post-treatment strategy opens a window for designing next generation MOF-related gas separation membranes.