Polyethylene glycol (PEG)-grafted metal-organic polyhedra (MOPs) provided defect-free mixed-matrix membranes via the solution-cast process. Compared to the neat PEG-based membranes, the membranes prepared with PEG-grafted MOPs showed enhanced gas permeation for carbon dioxide, while maintaining selectively toward methane, which could be ascribed to the distinctive self-organized nanostructure and core-shell architecture of PEG-grafted MOPs.