Tumor metastasis is attributed not only to the abnormalities of cancer cells, but also to changes induced by the interaction of cancer cells and surrounding cells/tissues. The host immune response to cancer cells may contribute to an increased incidence of tumor metastasis. Surgical removal of tumor tissues can trigger tumor recurrence and metastatic tumor growth in distant organs. An important class of molecules involved in these events is the reactive oxygen species (ROS), which have been identified as involved in not only to tumor metastasis but also most disease processes. ROS will contribute to various aspects of malignant tumors, including carcinogenesis, aberrant growth, metastasis, and angiogenesis. High-level ROS, which can be reached by several anti-cancer treatments, suppresses tumor metastasis by destroying cancer cells because of the oxidative nature of the molecules. On the other hand, sublethal levels of ROS can induce additional changes in DNA of tumor cells to make those cells malignant, stimulate the proliferation of cancer cells, and activate the expression of various molecules, some of which assist cancer cells to form metastatic colonies. Thus, a precise understanding how ROS are generated and involved in tumor metastasis will help us to design better strategies to overcome such life-threatening events.