Dual-layer composite membranes composed of Pebax 1657, different 1-Ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) contents and Polyethersulfone (PES) were fabricated by co-casting method, to investigate CO2/light gases separation, due to the high affinity of CO2 to Pebax and the ionic liquid (IL). Gas permeation test, which was performed at 25 °C and pressure range of 4–8 bar, indicated remarkable increment in CO2, CH4 and N2 permeance through membranes, especially CO2, as well as ideal CO2/CH4 and CO2/N2 selectivities, with IL increment. At 6 bar, CO2 permeance increased 152.97% meanwhile, at 8 bar, CO2/CH4 and CO2/N2 selectivities increased 59.22% and 28.64%, respectively, in the membrane containing 20 wt. % IL, which is the optimum IL content. In addition, CO2 permeance and selectivity increased with pressure. FTIR, XRD, DSC, SEM, dynamic frequency sweep, contact angle and tensile test results revealed the formation of defect-free dense Pebax/IL top-layer on porous PES sub-layer. IL addition led to formation of new bonds between the IL and Pebax which cause changes in the chain packing and crystallinity of Pebax and fabrication of more amorphous top-layer and more porous sub-layer. These results are confirmed by decrement in the mechanical properties of Pebax/IL casting solutions and composite membranes, by IL increment.