The　rational　design　of　mixed　matrix　membranes　(MMMs)　combining　appropriate　additives　and　industrial　process-able　polymers　offers　a　new　prospect　for　highly　efficient　membrane　separations.　However,　the　difficulty　in　their　fabrication　due　to　the　lack　of　processability　of　most　additives　and　the　formation　of　segregated　filler　domains　in　polymer　matrices　hampers　the　development　and　application　of　MMMs.　Herein,　we　present　two　new　zirconocene-based　metallocavitands　(ZRA　and　ZRT)　and　a　series　of　MMMs　fabricated　by　blending　ZRA,　ZRT,　and　MOC-1-NH2　(a　zirconocene-based　metal-organic　cage)　with　the　polyimide　(6FDA-DAM),　respectively.　It　was　found　that　ZRA　exhibited　high　thermal　stability　and　good　solubility　in　various　solvents,　which　can　be　homogeneously　mixed　with　polymers　to　prepare　ZRA-based　MMMs　without　any　indication　of　filler　agglomeration.　Compared　to　the　pure　6FDA-DAM　membrane,　a　low　ZRA-loading　MMM　(ZRA-2.2%)　exhibited　better　mechanical　properties　and　50%　improved　CO2　permeability　for　CO2/CH4　separation.　ZRA-2.2%　also　exhibited　a　good　performance　after　6　months　of　aging　and　significantly　alleviated　the　plasticization　effect　under　high　pressures.　These　results　suggest　the　great　potential　of　the　ZRA-2.2%　for　CO2/CH4　separation　and　that　the　simple　metallocavitands　may　serve　as　promising　additives　in　fabricating　MMMs　for　gas　separations.