Oxidative stress can initiate genetic and epigenetic alterations. Oxidative stress-induced genetic changes mainly involve that ROS attacks DNA, inducing various DNA changes like strand breaks, base modi?cations, DNA-DNA and DNA-protein cross linkages which are all strongly implicated in the initiation stage of tumorigenesis. In oxidative stress-induced epigenetic changes, one of the most important mechanisms is the change of DNA methylation levels: hypermethylation-induced transcriptional repression (in the case of tumor suppressor genes) or hypomethylation-induced activation (in the case of oncogenes), which is closely associated with tumorigenesis. In this paper, the current status of knowledge on the role of ROS-induced oxidative stress in altering the genetic and DNA methylation during tumorigenesis was introduced, and the developing tendency of tumor therapy was prospected.Cited