Equol a metabolite of daidzein has the strongest antioxidant activity among soybean isoflavones and its metabolites. It has two enantiomers. More and more studies have shown that the antitumor effect of soybean isoflavone depends on its metabolite equol. However the preventive effect of equol and its enantiomers on various types of tumors is not uniform and the underlying mechanism is not clear. Observational studies of equol and tumor outcomes suggest that dietary habits soy intake equol phenotype and ethnicity may be major contributors influencing the results. Animal studies to obtain the carcinogenic effects of R- and S -Eq in sufficient amounts and human intervention studies to directly address the effects of equol enantiomers on intermediate biomarkers of cancer risk may help to further define the impact of equol producers on phenotype and the mechanism of action of Eq itself. However it is certain that equol as a metabolite of soybean isoflavone has higher anti-tumor activity wider anti-tumor targets and lower toxicity than its precursors which plays an important role in protecting human health and preventing diseases. Subsequent studies should further explore the effects of different races different phenotypes and different doses of equol on the risk of tumorigenesis so as to provide a basis for rational application of equol.