Imidazolate framework ZIF-8 is modified via postsynthetic method using etheylenediamine to improve its adsorption performance toward CO2. Results show that the BET surface area of the modified ZIF-8 (ED-ZIF-8) increases by 39%, and its adsorption capacity of CO2 per surface area is almost two times of that on ZIF-8 at 298 K and 25 bar. H2O uptake on the ED-ZIF-8 become obviously lower compared to the ZIF-8. The ED-ZIF-8 selectivity for CO2/N2 adsorption gets significantly improved, and is up to 23 and 13.9 separately at 0.1 and 0.5 bar, being almost twice of those of the ZIF-8. The isosteric heat of CO2 adsorption (Qst) on the ED-ZIF-8 becomes higher, while Qst of N2 gets slightly lower compared to those on the ZIF-8 Furthermore, it suggests that the postsynthetic modification of the ZIF-8 not only improves its adsorption capacity of CO2 greatly, but also enhances its adsorption selectivity for CO2/N2/H2O significantly. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 21952206, 2013