AgO nanoparticles (NPs) were generated in ionic liquid 1-methyl-3-octylimidazolium tetrafluoroborate (Moim+BF4 .). The formation of Ag NPs was attributable to the favorable interaction between the surface of particles and counteranions of ionic liquid. The generated AgO nanoparticles in Moim+BF4 . were confirmed by TEM, and the average size was 40 nm. Coordinative interactions of AgO NPs with Moim+BF4 . were investigated by FT-Raman spectroscopy. The prepared Moim+BF4 ./Ag NPs composites were utilized for CO2 separation membrane, and consequently, CO2 separation performance for composite membranes was improved compared with neat Moim+BF4 .. The ideal selectivity for CO2/N2 was 23.4 with a CO2 permeance of 18.7GPU was observed, while the neat Moim+BF4 . membrane showed a selectivity of 15.9 and a CO2 permeance of 12.7GPU.