Membrane gas separation technologies are projected to attain a high efficiency for carbon dioxide (CO2) separation or capture from flue gasses. In this article, we report the CO2 permeability and CO2/N-2 selectivity of the mixed matrix membranes (MMMs) consisting of polymer and silica nanoparticles having gas permeable nano-space. The formation of gas transport channels by the channel-like assembly of the nanoparticles showed the dependency of the nanoparticle concentration and the CO2 permeability of the MMM significantly increased with an increase of the concentration without the decreasing CO2/N-2 selectivity. The CO2 permeability coefficient (P-CO2) reached up to 1920 Barrer (10(-10) cm(3)(STP) cm/(cm(2) s cm Hg)) without the significant decreasing gas selectivity (P-CO2/P-N2 = 23 for the MMM containing 25 wt% of silica nanoparticles modified with the second generation dendritic moieties. The importance of the surface-modified structures was investigated by comparing other silica nanoparticles without the dendritic structures. Furthermore, the ultrahigh CO2 permeation through the nano-space on the surface of the nanoparticles was first revealed by the Maxwell model calculation. The syntheses of the nanoparticles are cost-effective, and the nanoparticles with controllable nano-space can be easily synthesized by selection of the appropriate building blocks. Hence, MMMs composed of the nanoparticles having gas permeable nano-space have excellent potential for large-scale highperformance CO2 separation membrane.