A double-blind intervention trial was conducted in patients with oral mucosa leukoplakia using a mixed tea product developed by the authors. Fifty-nine oral mucosa leukoplakia patients, diagnosed by established clinical and pathological criteria, were randomly divided into a treated group (3 g mixed tea oral administration and topical treatment) and a control group (placebo and glycerin treatment). After the 6-month trial, the size of oral lesion was decreased in 37.9% of the 29 treated patients and increased in 3.4%; whereas the oral lesion was decreased in 10.0% of the 30 control patients and increased in 6.7%. At the same time, the incidence of micronucleated exfoliated oral mucosa cells in the treated group (5.4 per 1000 cells) was lower than that in the control group (11.3 per 1000 cells)(P < 0.01); whereas it was 1.4 per 1000 cells in 20 healthy subjects. The micronuclei and chromosome aberration rate in the peripheral blood lymphocytes showed the same results. In pathological examination, there were significant differences (P < 0.05) in the number and total volume of the silver-stained Nucleolar Organizer Regions (AgNOR) and the proliferating index of Proliferation Cell Nuclear Antigen (PCNA) in oral mucosa cell nuclei between the treated group and the control group which indicates that cell proliferation was decreased in the treated patients. The overall results provide some direct evidence on the protective effects of tea on oral cancer.