A method of controlling an X-ray in a computed tomography (CT) apparatus includes: acquiring scout images of an object; setting an imaging region of the object in the acquired scout images; determining an outline of transverse axes lengths of the imaging region based on the transverse axes lengths of the imaging region; controlling X-rays emitted toward the object by adjusting a distance between elements of a transverse collimator of the CT apparatus according to the determined outline; and reconstructing a cross-sectional X-ray image of the object based on X-ray projection data generated by detecting the controlled X-rays.