Novel copolyimides containing fluorine and naphthalene groups were synthesized from 4,4'-(Hexafluoroisopropylidene)diphthalic dianhydride (6FDA) and two diamines, 1,4-Bis(4-amino-2-trifluoromethylphenoxy) benzene (BATB) and 2,7-bis(4-aminophenoxy) naphthalene (BAPN) in various ratios via thermal imidization. The introduction of fluorinated groups into dianhydride improved transparency of copolyimides. All films are transparent. The structures and physical properties of copolyimides were characterized by FT-IR, DSC, TGA, XRD, UV-Vis and dielectric analyzer to study the effect of diamines on the physical properties of modified polymers. Copolyimides containing a higher percentage of BAPN have higher glass transition temperature and are less soluble in organic solvents. The copolyimide consisting of 50/50 mole% of BATB/BAPN has the lowest dielectric constant (2.60 at 1 KHz), reflecting the synergizing effect from both diamines.