HKUST-1 nanocrystals with large accessible surface areas and high porosity were functionalized with amine groups and incorporated into polymer matrices to form mixed-matrix membranes for application in CO2 capture. This gas permeation study reveals that pristine HKUST-1 nanocrystals are able to increase the gas permeabilities of polymer membranes with no noticeable effect on CO2/N-2 selectivity as a result of their high porosity and large pore dimensions. Contrarily, 20 wt% amine-functionalized HKUST-1 can improve the CO2/N-2 selectivity of polymer membranes by as much as 38%. As shown in a detailed analysis of the diffusivity-solubility, the amine-functionalized HKUST-1 suppressed N-2 solubility and diffusivity but enhanced CO2 diffusivity. This resulted in an increased CO2/N-2 perm-selectivity. Meanwhile, the pristine HKUST-1 nanocrystals increased the diffusivity and solubility of both species in a non-selective manner. These results indicate the feasibility of tuning the CO2 separation performance of mixed-matrix membranes by introducing amine functionality into the filler.