Poor dispersity of filler particles always limits their application in mixed matrix membranes (MMMs), increasing the difficulties in membrane fabrication and restraining the full exploitation of the intrinsic separation ability of the fillers. Herein, polyethyleneimine (PEI), a functional branched polymer, was covalently bonded on the outer surface of CAU-1 nanoparticles to implement increased colloidal stability and improved CO2 affinity for MMMs preparation. As a result, the dispersity of CAU-1 particles in solvent and MMMs was greatly improved after surface functionalization. Meanwhile, the grafted PEI polymer chains can serve as CO2 carriers simultaneously, which benefits the CO2 transport in MMMs. The prepared MMMs based on modified CAU-1 exhibit largely improved CO2/CH4 separation performance, with a CO2 permeability of 546 Barrer and a CO2/CH4 separation factor of 27.8 at 30 wt% filler loading, showing 165% and 50% increases respectively compared with the pristine polymer membrane. The modification approach presented here is expected to extend the functionality of MOFs and facilitate the development of MMMs. (C) 2018 Elsevier Ltd. All rights reserved.