Flexible PET/ITO (PolyEthylene Terephthalate/ Indium Tin Oxide) implantable electrode array for spinal cord stimulation and retina prosthesis have been developed. The electrode array is fabricated on a thin PET/ITO substrate and encapsulated with insulating material, SU-8. The PET substrate made electrodes flexible so that they could shape to contoured tissues. A layer of gold on the stimulation sites served to reduce the electrode/tissue interface impedance. Prototypes of 1 × 8 and 3 × 8 electrode arrays are fabricated for monophasic and biphasic stimulation of spinal cord respectively. The exposed electrode dimensions are 3 mm² for monophasic and 6 mm² for biphasic stimulation with 100 μm of interconnection paths. The prototype of 4 × 4 electrode arrays were also fabricated with the same process for retinal prosthesis with exposed electrode diameter of 125 μm. To verify the functionality of the subdural electrodes, the electrochemical impedance spectroscopy was measured. The electrode/tissue impedance was 500 Ω at 1KHz for 3 mm² area.