A series of transparent, 100 m thick, polyimide (PI) composite films were prepared by a sol-gel reaction using a thermal imidization process between poly(amic acid) (PAA) and various amounts of tetraethyl orthosilicate (TEOS) in N,N-dimethylacetamide. The PAA were prepared using 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2'-bis(trifluoromethyl)benzidine (TFMB) and 3,5-diaminobenzoic acid (DABA). The chemical structure and thermal stability of the PI/silica composite films were examined in detail by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The PI/silica composite films showed high thermal stability, and the degradation behavior was found to be dependent on the amount used. In addition, the optical and thermo-mechanical properties of the films were investigated, including the haze values, yellow indices and in-plane coefficients of thermal expansion (CTE). The CTE values of the PI/silica composite films were 52.0 - 43.5ppm/degrees C, and were dependent on the amount of TEOS used. The haze values and yellow indices increased with increasing TEOS concentration. Optically transparent PI/silica composite films were obtained with a TEOS content of up to 20wt%.