We suggest a facile synthesis method of copper nanoparticles, and this process is facilitated by 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4-) which was employed to dissociate the copper metal into nanosized copper particles. The formation of copper nanoparticles was confirmed by UV-vis spectrum analysis and transmission electron microscope (TEM) imaging, and the interaction of BMIM+BF4- with the copper metal was confirmed by Fourier transform (FT)-Raman spectroscopy. Moreover, the fabricated copper nanoparticles could be used as an olefin carrier for propylene/propane separation. While BMIM+BF4-/Cu (stirred for 0 h) nanocomposite showed a selectivity of 1.1 and a permeance of 4.7 GPIJ, the separation performance of the BMIM+BF4-/Cu (stirred for 24 h) nanocomposite membrane was significantly improved; the selectivity was found to be 5.2 with a mixed gas permeance of 4.0 GPU.