In this paper we described a modified fluorometric analysis of DNA unwindingg method for the detection of DNA strand breaks in cells. Some factors which might influence the results were also investigated, and the effect of oxygen on the induction of DNA strand breaks and its repair in γ-irradiated mammalian cells was observed. It was shown that a liner relationship existed between the dose and the extent of DNA damage. OER of L,7712, cells was 3.0. while OER of 3T3 cells were 1.37 and 1.01 under two different irradiation conditions. The differences between them were an interested and significant question. Incubated at 37℃ under oxic condition, the hypoxically irradiated 3T3 cells rejoined their strand breaks more rapidly and efficiently than the oxically irradiated cells did. Post-irradiation hypoxia could inhibit the rejoining of strand breaks in the hypoxically irradiated 3T3 cells.