To modification of polyvinylchloride membranes for separation of methane and carbon dioxide gases, 1-vinyl-3-butyilimidazolium halide with three different anions (bromide, [Br](-), thiocyanate, [SCN](-), and tetrafluoroborate, [BF4](-)) was grafted onto the polyvinylchloride by free radical graft copolymerization. The prepared membranes were characterized by Fourier transform infrared spectroscopy (FTIR) and elemental analyzer (CHN) technique. The permeability of methane and carbon dioxide gases was performed using constant pressure/variable volume method. The effects of variation of anions, feed pressure, and temperature on the membrane performance were investigated. The results indicate that the 1-vinyl-3-butyilimidazolium with [SCN](-) anion has better performance to separation of carbon dioxide and methane gases. Also, the enhancement of temperature led to decrease the selectivity of membrane for carbon dioxide/methane gases; whereas, the selectivity of membranes was increased with increasing feed pressure. Also, to confirm the obtained results, the interaction energy between carbon dioxide gas and ionomer membrane with different anions was investigated by density functional theory calculations.