The mercury is among the most highly bioconcentrated toxic trace metals. Many national and international agencies and organisations have targeted mercury for the possible emission control. The mercury toxicity depends on its chemical form, among which alkylmercury compounds are the most toxic. A human cervix uterus cancer cell line HeLa cells was employed to investigate the effect of the toxic heavy metal mercury (Hg) and ionizing radiation. In the in vitro comet assays for the genotoxicity in the HeLa cells, the group of Hg treatment after irradiation showed higher DNA breakage than the other groups. The tail extent moment and olive tail moment of the control group were $4.88{pm}1.00;and;3.50{pm}0.52$ while the values of the only Hg treatment group were $26.90{pm}2.67;and;13.16{pm}1.82$, respectively. The tail extent moment and olive tail moment of the only 0.001, 0.005, 0.01 Hg group were $12.24{pm}1.82,;8.20{pm}2.15,;20.30{pm}1.30,;12.26{pm}0.52,;40.65{pm}2.94;and ;20.38{pm}1.49$, respectively. In the case of Hg treatment after irradiation, the tail extent moment and olive tail moment of the 0.001, 0.005, 0.01 Hg group were $56.50{pm}3.93,;32.69{pm}2.48,;62.03{pm}5.14,;31.56{pm}1.97,;72.73{pm}3.70;and ;39.44{pm}3.23$, respectively. The results showed that Hg induced DNA single-strand breaks or alkali labile sites as assessed by the Comet assay. It is in good agreement with the reported results. The mercury inhibits the repair of DNA. The bacterial formamidopyrimidine-DNA glycosylase (Epg protein) recognizes and removes some oxidative DNA base modifications. Enzyme inactivation by Hg (II) may therefore be due either to interactions with rysteine residues outside the metal binding domain or to very high-affinity binding of Hg (II) which readily removes Zn (II) from the zinc finger.