DNA methylation plays a crucial role in eukaryotes' growth and development, which is catalyzed by DNA methyltransferase (MTase). DNA MTase can transfer the methyl group to the C-5 position of cytosine in eukaryote from the methyl group donor of S-adenosylmethionine (SAM). Here, we developed a sensitive electrochemical method for assay of DNA methyltransferase (MTase) activity and screening of DNA MTase inhibitor without labeling processes. This strategy is mainly based on the change of reduction peak current of ferrocenecarboxylic acid (FcA), which is conjugated to the biosensor as electrochemical activity probe. The reduction current of FcA was enhanced with increasing DNA MTase concentration from 0.1 to 70 unit/mL, where the linear range of the concentration was from 0.1 to 1 unit/mL and 1 to 50 unit/mL with the detection limit of 0.045 unit/mL (S/N = 3). The investigation results demonstrated that the activity of DNA MTase depended on MTase concentration and incubation time. Inhibition experiment indicated that fisetin had good inhibition activity on DNA MTase in the presence of catechol-O-methyltransferase. This electrochemical method had the potential application in the screening of DNA MTase inhibitor, provided valuable information for anti-cancer drug research and also for cancer therapy. (C) 2012 Elsevier Ltd. All rights reserved.