Objective: To study transcription level and catalytic activity of DNA methyltransferase (Dnmts) in lung cancer cell line A549 exposed in 5-Aza-CdR(5-Aza-2’-deoxycitydine),and the role of Dnmts in the process. Methods:A549 cells were exposed in the 5-Aza-CdR for demethylation. Dnmts mRNA levels were determined by semi-quantitative RT-PCR. The catalytic activity of Dnmts was detected by enzyme colorimetric determination kit. The apoposis and changes of cell cycles were observed by flow cytometry(FCM),and MTT assay was used to study the cell proliferation. Results:The transcription level of the drug group and control group were Dnmt1(1.23 ± 0.253;1.15 ± 0.166), Dnmt3b(0.760 ± 0.164;0.649 ± 0.181),No significant difference in quantity of Dnmts mRNA can be oberserved between the two groups (P > 0.05). Catalytic activity of the drug group and control group were Dnmt1 (0.195 ± 0.030;0.153±0.041),Dnmt3b(0.172 ± 0.029;0.116 ± 0.050). Catalytic activity decreased compared with the control group (P < 0.05). FCM recommended 5-Aza-CdR arrested cell cycle at G1/G0,the rates of apoptosis of the two groups were 0.62 ± 0.56;7.60 ± 1.92,and the rate of apoptosis of drug group was higher than the control group (P < 0.01). Cell proliferation of drug group was inhibited according to the results of MTT. Conclusion: 5-Aza-CdR has cytostatic agent in A549 cells cultured in vitro,and accelerates its apoptosis. The inhibition of Dnmts catalytic activity can account for demethylating agent of 5-Aza-CdR in A549 cells,not the Dnmts transcription level.