A method of calibrating a first threshold voltage that is a reference of X-ray detection for each unit cell of a plurality of unit cells of an X-ray detector may comprise detecting an X-ray by using a plurality of second threshold voltages for each of a plurality of X-rays having spectra at different energy levels; determining a correspondence relationship between energies having a maximum intensity in the spectra of X-rays and third threshold voltages at which a maximum number of photons having a same energy intensity are detected; and/or calibrating the first threshold voltage based on the determined correspondence relationship.