In order to investigate the mechanism of Quercetin on radio-sensitivity of human Uterine Cervix Cancer HeLa cells, HeLa cells were cultured in different concentrations of Quercetin and different doses of irradiation. The clonogenic assay was used to observe the cell servival rate. The repair of DNA double-strand breaks and effect of Quercetin combination of radiation on the cell cycle were detected by flow cytometry. The results show that the radio-sensitivity of Quercetin on HeLa cells was obvious and the unrepaired DSBs after irradiation increased, but did not decrease G2 / M cell cycle arrest. From this it can be inferred that the effect on HeLa cell radio-sensitivity may be related to the inhibition of the repair of DNA double-strand breaks induced by Quercetin, but it dose not reveal a significant relation with the cell cycle and G2/M arrest.