A thin film transistor array panel for an X-ray detector includes a dummy pixel including a photo diode and a TFT for detecting leakage current. The photo diode includes first and second electrodes (178,195) facing each other and a photo-conductive layer (800) disposed between the first electrode and the second electrode. The TFT includes a semiconductor layer (150), a gate electrode (123), a source electrode (173) connected to a data line, a drain electrode (175) connected to the photo diode. The dummy pixel further includes a light blocking layer (196) for blocking light incident on the photo diode. Alternatively, the semiconductor layer is disconnected between the source electrode and the drain electrode.