Copolyimides containing pendant trifluoromethyl (CF3) groups were synthesized from 4,4′- (hexafluoroisopropylidene)diphthalic anhydride (6FDA) and bis[4-(3-aminophenoxy)phenyl]sulfone (BAPS) with various concentrations of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BAPP) to poly(amic acid)(PAA), followed by thermal imidization. These copolyimides were readily soluble in N,N′-dimethylacetamide (DMAc) and could be solution.cast into a flexible and tough film. The thermomechanical properties, morphology and an optical transparency of the copolyimide films were determined using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide angle X-ray diffraction (XRD), scanning electron microscopy (SEM), universal tensile machine (UTM), and a UV-Vis spectrometer. The cast copolyimide films exhibited high optical transparency with a cut-off wavelength (λo) of 275∼319 nm in UV-vis absorption and a low yellow index(YI) value of 3.65∼10.37. The thermo-mechanical properties of copolyimide films were enhanced linearly with increasing a BAPP content. In contrast, the optical transparency of the copolyimide films was found to get worse with increasing a BAPP content.