A method of fabricating an image TFT array of a direct X-ray image sensor includes forming a first transparent conductive layer on a substrate; forming a gate line including a gate electrode, a common line, and a common electrode jutting out from the common line; forming an insulation layer; forming a semiconducting island on the insulation layer in the transistor region; forming a first via hole for the common electrode; forming a data line and a source electrode and a drain electrode; forming a passivation layer and a second via hole penetrating the passivation layer for the source electrode; forming a second transparent conductive layer as a top electrode. The insulation layer is formed on the first transparent conductive layer to serve as a dielectric layer of a capacitor before the TFT structure formed and can be formed at a relatively high temperature.