Despite significant advances in cancer therapeutic modalities, available anti-tumor drugs display limited efficacy and sometimes carry a risk of severe adverse side effect. Therefore, it is important to identify and develop cancer chemopreventive agents without toxicity. Epidemiological examinations in human populations and experimental rodent studies provide evidence that certain types of phytochemicals suppress development and growth of cancers at various organ sites. A number of clinical investigations have been also conducted and shown that dietary phytochemicals are able to inhibit tumorigenesis, indicating several phytochemicals are regarded as cancer chemopreventive agents. Epigallocatechin-3-gallate (EGCG), the major catechin in green tea, is considered as the most biologically active constituent in drinking tea with respect to inhibiting cell proliferation and inducing apoptosis in cancer cells. EGCG appears to directly target receptor tyrosine kinases with inhibiting activation of the receptors and down-stream signaling pathways. In addition, EGCG has improving effects against obesity and insulin resistance, which leads to suppressing the development of obesity-related malignancies. Other polyphenolic phytochemicals, including curcumin and resveratrol, are also reported to exert anti-cancer effect. In this review article, we summarize the potential of dietary phytochemicals as anti-cancer drugs and those possible mechanisms against cancer, especially chemopreventive effect of green tea catechins.