In this work, a series of mixed matrix membranes (MMMs) consisting of Pebax (R) 1657 as the main polymer matrix, poly(ethylene glycol) (PEG) derivatives with low molecular weights as additives, and graphene oxide (GO) with different loadings as a nanofiller, were prepared for CO2/N-2 separation. Changes in the amorphous and crystalline regions of Pebax (R) / PEG blend membranes and in the free volume of GO-embedded MMMs were estimated based on differential scanning calorimetry (DSC) thermograms. The fractional free volume and density of the Pebax (R) /PEG blend membranes were investigated according to additive models. Particularly, blending poly (ethylene glycol) methyl ethyl acrylate (PEG-MEA) with the Pebax (R) matrix significantly improved CO2 permeability without sacrificing high CO2/N-2 selectivity. Incorporating GO nanosheets up to 0.3 wt% into the Pebax (R) / PEG-MEA matrix was found to have a considerable impact on increasing CO2/N-2 selectivity, due to largely in part to an increase in the CO2 solubility coefficients. Upon a further increase in GO loading, both CO2 permeability and CO2/N-2 selectivity decreased as a result of increasing tortuosity for gas diffusion and also GO aggregation at high GO loading content. More importantly, the Pebax (R) /PEG-MEA blend membrane with the optimized loading of GO, i. e., 0.3 wt% indicated outstanding anti-CO2 plasticization resistance up to 10 bar as well as long-term stability over 100 days without significant deterioration of CO2/N-2 separation performance.