Example embodiments are directed an X-ray detector including an oxide semiconductor transistor. The X-ray detector including the oxide semiconductor transistor includes an oxide semiconductor transistor and a signal storage capacitor in parallel to each other on a substrate. The oxide semiconductor transistor includes a channel formed of an oxide semiconductor material, and a photoconductor. A pixel electrode and a common electrode are formed on opposite surfaces of the photoconductor. The channel includes ZnO, or a compound including ZnO and at least one selected from a group consisting of gallium (Ga), indium (In), hafnium (Hf), and tin (Sn).