A graft copolymer of poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM) was synthesized via atom transfer radical polymerization (ATRP), and fumed silica (SiO 2 ) nanoparticles were modified by grafting POEM via a three-step synthetic approach. The resultant graft copolymer and modified SiO 2 nanoparticles were solution blended to prepare PVC-g-POEM/SiO 2 -POEM nanocomposite membranes. Uniform distribution of SiO 2 -POEM nanoparticles in the membranes and microphase-separated morphology were confirmed by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). X-ray diffraction (XRD) analysis revealed increased randomness in the amorphous phase of PVC-g-POEM and continuous decrease in the interchain d-spacing with increased SiO 2 -POEM content, indicating a more closely packed nanocomposite membrane structure. The mechanical properties of PVC-g-POEM/SiO 2 -POEM membranes were superior to those of PVC-g-POEM, resulting from strong interfacial interactions in the membranes due to the segregation and entanglement of the POEM chains. In addition, the permeation performances of the PVC-g-POEM/SiO 2 -POEM nanocomposite membranes were somewhat better than those of PVC-g-POEM, which showed excellent CO 2 /N 2 and CO 2 /H 2 separation capabilities. For example, the CO 2 permeability and CO 2 /N 2 selectivity of PVC-g-POEM/SiO 2 -POEM (2%) at 35 o C were 200 Barrer and 43, respectively. Thus, we successfully prepared membranes that deliver high permeation properties, as well as good mechanical properties.