Graphene oxide (GO) can be used as a membrane material itself or a nanofiller to enhance gas separation performance of polymer membranes. Since GO has high CO2 affinity due to some polar groups, particularly GO membranes or GO/polymer membranes have been extensively studied for CO2 separation. Although ultrathin GO membranes show outstanding CO2 separation properties, the gas permeance through GO membranes is still low owing to high tortuosity caused by high aspect ratio of GO sheets. In this study, mixed-matrix membranes consisting of modified GO (as a dispersed phase) and high permeable polymer were prepared by combining each advantage of GO and high permeable polymer for improving gas separation performance. Both single-gas and mixed-gas permeation experiments were conducted with or without humidified feeds for post-combustion CO2 capture.