Abstract: ［Abstract］Objective　To study the reactive oxygen level and the expression of anti-apoptotic protein Bcl-2 and pro-apoptotic protein Bax after treatment of DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine （5-Aza-2'-dc） and paraquat in V79 cells．Methods　Cultured V79 cells were divided into 5-Aza-2'-dc treatment group （group A）， paraquat treatment group（group B），5-Aza-2'-dc and paraquat treatment group （group C， V79 cells were pretreated with 5-Aza-2'-dc for 12h followed by exposure to paraquat for 12h）and control group （group D）. Reactive oxygen level in V79 cells was measured by DCFH-DA flow cytometry and expression of Bcl-2 and Bax was detected by Western blot．Results　Reactive oxygen levels and expression levels of Bcl-2 and Bax in V79 cells were significantly different（P＜0.05） in 5-Aza-2'-dc and paraquat treatment group（group C），compared with 5-Aza-2'-dc treatment group （group A）， paraquat treatment group（group B） and control group（group D）. Expression levels of Bcl-2 and the ratio of Bcl-2 and Bax were lower while reactive oxygen levels and expression levels of Bax were higher in group C than in groups A， B and D. Conclusion 5-Aza-2'-dc regulates DNA methylation by the imbalancing the reactive oxygen metabolism and apoptosis， thus up-regulating the toxic effect of paraquat on V79 cells.