It is well established that aberrant gene regulation by epigenetic mechanisms can develop as a result of pathological processes such as cancer. Methylation of CpG islands is an important component of the epigenetic code and a number of genes become abnormally methylated during tumorigenesis. Some bioactive food components have been shown to have cancer inhibition activities by reducing DNA hypermethylation of key cancer-causing genes through their DNA methyltransferase (DNMT) inhibition properties. The dietary polyphenols, (-)-epigallocatechin-3-gallate (EGCG) from green tea, genistein from soybean and possibly isothiocyanates from plant foods, are some examples of these bioactive food components modulated by epigenetic factors. The activity of cancer inhibition generated from dietary polyphenols is associated with gene reactivation through demethylation in the promoters of methylation-silenced genes such as p16INK4a and retinoic acid receptor β. The effects of dietary polyphenols such as EGCG on DNMTs appear to have their direct inhibition by interaction with the catalytic site of the DNMT1 molecule, and may also influence methylation status indirectly through metabolic effects associated with energy metabolism. Therefore, reversal of hypermethylation-induced inactivation of key tumor suppression genes by dietary DNMT inhibitors could be an effective approach to cancer prevention and therapy. In this analysis, we focus on advances in understanding the effects of dietary polyphenols on DNA methylation modulation during the process of cancer development, which will offer exciting new opportunities to explore the role of diet in influencing the biology of cancer and to understand the susceptibility of the human epigenome to dietary effects.