Effects of ion beam irradiation defects on electrical and optical properties of CdZnTe (CZT) crystals were studied. CZT crystals grown by the modified vertical Bridgman method were irradiated by Ar heavy ions with fluences ranging from 10¹⁴ cm^-2 to 10¹⁵ cm^-2. Results show that IR transmittance spectra vary from the high-straight type (before radiation) to the ascending type (after radiation), and light absorption by radiation induced free charge carriers occurs significantly in the mid-infrared light range. Current-voltage (I-V) characteristic curve of the irradiated CZT crystals becomes extremely asymmetrical and current increases sharply with the increase of negative voltage due to single-surface irradiation in the near-surface area. Hall measurements show that the net carrier concentration largely increases from ~10⁶ cm^-3 (before radiation) to ~10¹⁶ cm^-3 (after radiation), and the conduction type remains unchanged, since donor levels are considered to be the dominant among all the radiation-induced crystal defects.