It is reported that Irisquinone, an active component from Chinese herb Iris pallasii Fischer, could modify the radiosensitivity of the tumor cells in cancer radiotherapy. By means of alkaline elution method modified in our laboratory, the effect of Irisquinone on γ-ray induced DNA single strand breaks and their rejoining in S-180V cells was studied. The results showed that this compound could inhibit both the processes of normal DNA synthesis and the rejoining of DNA single strand breaks. There was no significant difference between the effects under aerobic and hypoxic conditions.