The development of a high power and high energy density supercapacitor would enable a technology for future space missions due to the need of ultra-lightweight devices, relatively high energy, and high power density with multiple charging cycles. This gives rise to the nanostructured supercapacitors that could provide an operational regime that enhances current technology for making such sorts of devices. In this work, we have fabricated thin-film reduced graphene oxide (rGO) electrodes for ultra-lightweight supercapacitor by drop-casting technique on polyethylene terephthalate (PET) substrate. With an application of white light illuminated under a certain power on such the rGO thin-film electrodes, their sheet resistance was decreased gradually, which indicate the increasing of free charged carriers in the rGO film. After sandwiching two identical rGO film electrodes of about 4 cm2 with an electrolyte gel, a capacitor is formed with silver epoxy contacts. The capacitance of such a device was estimated using an RC (resistor-capacitor) circuit with a variety of different bias resistors. We found its capacitance of our rGO capacitors in order of micro Farads with light illumination, which is comparable with the commercial ones. This research should give an understanding of capacitance enhancement by photo-excited charge carriers in such a nanostructured device.