Our group synthesized mixed matrix membranes (MMMs) embodying metal-organic framework ZIF-11 with extremely narrow pore-openings (~3.0 Å of aperture windows) as the nano-fllers for CO2/CH4 separation. The MMMs with filler loading of 24 wt% showed a remarkable improvement in the separation factor of CO2/CH4 as well as CO2 permeability, corresponding to an improvement of 43.5% and 99.5% compared with that of native polymer membranes, respectively. Moreover, in-depth analysis of gas transport properties of the membranes showed that the restricted pore-openings of ZIF-11 performed as satisfied molecular sieving pathways of CO2 but crowded CH4. The optimal CO2/CH4 diffusion selectivity of MMMs increased by 160%. This work has been published in Journal of Membrane Science, 2020, 601,117880.