An X-ray imaging apparatus and a method of updating a pixel map correct a bad pixel of an X-ray transmission image. An X-ray transmission image is generated by detecting an X-ray penetrating an object, and it is determined whether a difference between a value of each one of the pixels forming the X-ray transmission image and a value of a nearby pixel adjacent to the each one of the pixels is equal to or greater than a reference value. A pixel having a value representing a difference equal to or greater than the reference value is determined as a candidate for a bad pixel. A message is displayed which requests a selection of whether to determine if the candidate is a bad pixel. The pixel map is updated by reflecting the determined bad pixel on the pixel map.