Polyetherimide membranes (PEIs) are made by solution-casting method. As a casting solvent, N-methyl pyrrolidone (NMP) was used for membrane solutions. For the purpose of investigating the effects of the membranes on gas separation efficiency, fumed silica nanoparticles and poly(ethylene glycol) (Mw=6000 g/mol) were added to the polymer solution. Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) were used to analyze PEI membranes. To investigate the membrane morphologies, scanning electron microscopy (SEM) was used to monitor the morphology of the membrane cross-sections. CO2 and CH4 gas permeability were measured to evaluate the performance of PEI-based membranes for gas separation. In SEM analysis, pure PEI membrane had dense, symmetrical structures. As a result of adding fumed silica nanoparticles to PEI membrane structure, finger-like voids were formed. An increase in fumed silica from 1% to 15% by weight led to a sponge-like dense asymmetric membrane structure observed by SEM. In the experiments, PEI-0-15 demonstrated the highest CO2/CH4 selectivity (α=36.483 and PCO2=0.6967 Barrer).