1. An X-ray imaging apparatus comprising: an X-ray generator configured to irradiate the X-rays onto an object; an X-ray detector configured to detect the X-rays transmitted through the object, and to generate an image frame; and an image processor configured to separate a first tissue image having a small amount of motion of a tissue, the small amount of motion being smaller than a threshold amount of motion, and a second tissue image having a large amount of motion of a tissue, the large amount of motion being larger than the small amount of motion, from a plurality of image frames about a plurality of tissues which are classified according to the motions of tissues, and to extract the separated first and second tissue images, and to correct the second tissue image according to the large amount of motion of the tissue.