Hyperthermia, either alone or in combination with radiation, in the treatment of cancer is an area of considerable interest.,In this essay, we used our modified millipore filter alkaline elution-fluorometric method to observe the effect of hyperthermia on radiation-induced DNA single strand breaks and rejoining in L,7712, cells and to clarify the mechanisms by which hyperthermia potentiates the cell killing effect of radiation.,The results showed that hyperthermia alone for 1 hr at temperature up to 42℃ did' not produce any measurable DNA strand breaks. Whereas at higher temperature exceeding 42℃ DNA strand breaks increased dramatically with the rise of heating temperature. Thus we suggested a new concept of "critical energy", which depended on both parameters of heating temperature and heating time at the same temperature. The effect of preirradiation hyperthermia mainly to increase DNA radiation-induced damage, but postirradiation hyperthermia caused the inhibition of rejoining of DNA strand breaks.