A method, performed by a mobile X-ray detector, of processing an X-ray image, including generating the X-ray image of an object by detecting an X-ray transmitted through the object and converting the detected X-ray into an electrical signal; detecting a power supply stoppage which prevents transmission of the generated X-ray image from the mobile X-ray detector to a workstation; based on the detecting of the power supply stoppage, storing the X-ray image in a nonvolatile memory inside the mobile X-ray detector; and after storing the X-ray image, deactivating the mobile X-ray detector.