DNA strand breaks are the main biological effect of ionizing radiation. Single strand break is induced by a single hit of irradiation and thus has linear dose response curve. Double strand break can be formed not only by a single hit, but also by the overlap of two close single strand breaks located in the complemental strands. The dose response curve of double strand breaks is linear -quadratic. DNA strand breaks can be detected quantatively by various physicochemical methods. Cells are able to repair both single and double strand breaks. The mechanisms of repair, however, are not so clear as those of other repair models, excision repair, for example. The unrepaired strand breaks are lethal. Single strand repair is error-free. The biological effects of repaied double strand kreaks remain unclear thus far. Dose rate has plays important role on the biological effects of ionizing radiations.