The effects of incorporation of fluorinated alkyl side groups into polyimide membranes were investigated in terms of their physical and gas permeation properties. Four polyimides with fluorinated side groups and four polyimides without the side groups were prepared by polycondensation of 2-(perfluorohexyl)ethyl-3,5-diamino benzoate (PFDAB) and m-PDA with four aromatic dianhydrides(6FDA, ODPA, BTDA and PMDA), respectively. It was found that the incorporation of fluorinated side groups into the polyimide membranes decreased their T_gs, surface free energies, solubility parameters and fractional free volume(FFV)s and therefore, enhanced the permeabilities for CO₂, O₂, N₂ and CH₄ gases but reduced the selectivities for CO₂/CH₄, O₂/N₂, CO₂/N₂ gas pairs depending upon the structure of dianhydride monomers.