It is reported that the water extract of tea as well as tea polyphenols and tea pigments have strong antioxidant properties, which are considered as the major mechanism of the protective effects of tea on cancer and cardiovascular diseases. It has been shown that tea (including tea polyphenols and tea pigments) could induce the antioxidant enzyme activities, regulate phase I and II metabolic enzymes, inhibit the metabolic activation of carcinogens, reduce the formation of carcinogen DNA adducts, inhibit the expression and replication of oncogenes, directly affect the activity of transcription factors, and block the initiation of carcinogenesis, so as to eliminate and alleviate the damage to cellular communication caused by oxidative stress, and eventually prevent the abnormal proliferation of tumor cells. The mechanisms of the protective effects of tea on cardiovascular diseases are possibly related to its effects on the inhibition of lipid oxidation and quenching oxygen and hydroxy free radicals. It was demonstrated that tea pigments could inhibit the oxidation of LDL cholesterol and the adhesion of blood vessel endothelium cells, lower endothelin levels, enhance GSH-PX activities, prevent from blood coagulation and platelet aggregation, and facilitate fibrinogen dissolution, so as to prevent atherosclerosis of coronary arteries.