Stomach and esophageal cancers have the highest mortality rate in China. The incidence of these cancers is closely related to diet and nutrition. Many mutagens and carcinogens have been isolated and identified in foods consumed in some high‐rate areas of esophageal and stomach cancers. Upon screening more than 100 different fruits, vegetables, and beverages in China, it is shown that green tea, onion, ginger, eggplant, coriander, banana, and orange skin are strong inhibitors of mutagenesis. Polyphenol components isolated from green tea showed strong antioxidant activity. The green tea antioxidant inhibited backward mutagenicity of AFB1and BaP inSalmonella typhimurium. It also inhibited gene forward mutation and chromosome aberrations in V79 Chinese hamster cells treated by AFB1and BaP. When the green tea antioxidant was given to mice with drinking water or topically applied to mouse skin, it inhibited the development of skin papilloma induced by DMBA/TPA. It was found that green tea antioxidant was a strong free‐radical trap.