It has been accepted that the separation performance for olefin/paraffin mixtures has been improved through composite membranes containing silver nanoparticles (Ag NPs) dispersed in a common polymeric matrix when their surface is positively charged by electron acceptor. Here, a correlation between the surface positive charge density, represented by the binding energy, on Ag NPs and the separation performance was explored. The binding energy of Ag atom was reached a maximum value as increasing the concentration of an electron acceptor such as nitroaromatic compounds (NACs). At the highest binding energy, the highest gas separation performance was also obtained: the remarkably high mixed-gas selectivity above 150 for a 50/50 (v/v) propylene/propane mixture and the propylene permeance about 0.9 GPU. The correlation between the surface positive charge density and the gas separation performance was observed to be linear regardless of the electron acceptor used. This correlation thus suggests that the major determinant of the olefin/paraffin gas separation performance is not the chemical characteristics of electron accepting compounds, but the induced surface positive charge density on Ag NPs. Therefore, it is concluded that surface charge density of Ag NPs is a key factor in determining the separation performance of facilitated olefin transport membranes.