A thin film transistor (TFT) array substrate for an X-ray detector and a method of fabricating the same are provided. The TFT array substrate includes a substrate, a gate line formed on the substrate, a data line crossing the gate line, a thin film transistor including a gate electrode, a source electrode, and a drain electrode, a first electrode connected to the drain electrode, a passivation layer formed over the gate line, the data line, the thin film transistor and the first electrode, a photoconductor formed over the passivation layer and connected to the first electrode, and a second electrode formed on the photoconductor.