Abstract: Selenium is an essential trace element for human health and has a variety of functions, such as antitumor effect, anti-oxidation, detoxification, and immune regulation. Selenium is closely related to tumorigenesis and selenium deficiency could lead to increased morbidities of various cancer, including colorectal, prostate, lung, esophageal cancers and hepatocelluar carcinoma. Selenium in the human body is incorporated into selenoproteins for its functioning and the polymorphism of selenoprotein genes is associated with tumor development. Selenoproteins can be used as biomarkers for molecular typing and the prognosis of tumor. Selenium-containing compounds can inhibit tumor growth, and enhance the efficacy of antitumor drugs. This paper aims to review the progress in researches on antitumor effect of selenium and its mechanism and to address the importance of selenium supplementation for improving health and preventing tumors.