When cells or DNA is exposed to ionizing radiation, the radicals produced in the irradiated sample will modify the base-pair region of the double strands. Effects of 1,2-dihydroxy-9,10-anthraquinone (DHA) and its Cu(II) complex on the radiation-induced modification of double-strands in calf thymus DNA were studied using ethidium bromide as a fluorescent probe. Our results show that the Cu(II)-DHA complex is more efficient in modifying the base-pair region in double-stranded DNA compared to free DHA.