There is provided a nanocomposite membrane comprising an Ag-nanoparticle/polymer nanocomposite, in which the Ag-particles are uniformly dispersed in the polymer matrix, and a support membrane for supporting the nanocomposite, as well as a process of preparing said membrane. The nanocomposite membrane of the present invention comprising a neutral Ag-nanoparticle as an olefin carrier, which is chemically stable, has excellent long-term operation performance characteristics as well as high selectivity and permeability. Thus, it can be advantageously used for the separation of olefin from an olefin/paraffin mixture.