In order to study the preventive effect of tea on DNA damage induced by cigarette smoking and to provide further evidence on the protective effects of tea in human cancer, a 6-month, double-blind and randomized placebo-controlled intervention trial was carried out. The effect of tea on micronuclei frequency in exfoliated oral buccal mucosa cells in 36 oral leukoplakias of smokers was investigated. A kind of mixed tea given in an oral dose of 3 g/day and a concentration of 0.1% smeared on mucosa lession three times a day for 3 months and 6 months, significantly decreased the micronuclei formation in exfoliated oral buccal mucosa cells in subjects with oral leukoplakias. In contrast, there was no change in the micronuclei frequency after 6 months in the placebo group. The results indicate that mixed tea may reduce the oral cancer risk by preventing DNA damage in oral leukopiakias induced by cigarette smoking.