Graphene oxide (GO)-polyether block amide (PEBA) mixed matrix membranes were fabricated and the effects of GO lateral size on membranes morphologies, microstructures, physicochemical properties, and gas separation performances were systematically investigated. By varying the GO lateral sizes (100-200 nm, 1-2 mu m, and 5-10 mu m), the polymer chains mobility, as well as the length of the gas channels could be effectively manipulated. Among the as-prepared membranes, a GO-PEBA mixed matrix membrane (GO-M-PEBA) containing 0.1 wt % medium-lateral sized (1-2 mu m) GO sheets showed the highest CO2 permeation performance (CO2 permeability of 110 Barrer and CO2/N-2 mixed gas selectivity of 80), which transcends the Robeson upper bound. Also, this GO-PEBA mixed matrix membrane exhibited high stability during long-term operation testing. Optimized by GO lateral size, the developed GO-PEBA mixed matrix membrane shows promising potential for industrial implementation of efficient CO2 capture. (C) 2016 American Institute of Chemical Engineers