Dietary flavonoids have attracted a great deal of attention as agents for preventing estrogen-related diseases, such as postmenopausal symptoms, and for reducing the risk of estrogen-dependent cancer. Kaempferol is one of the most commonly found dietary phytoestrogen. The aim of this study was to investigate the estrogenic and/or antiestrogenic effect of kaempferol, which can confirm its potency as a preventive agent against estrogen-related diseases. Kaempferol has both estrogenic and antiestrogenic activity, which are biphasic response on estrogen receptor. The estrogenic activity of kaempferol induced via ER-mediated pathway depending on E2 concentration (≤10-12 M). Kaempferol (10-5 M) also caused antiproliferative effect on MCF-7 cell in the presence of E2 (10-11 M) and restored to the addition of excess E2 (10-7 M), which confirms that antiproliferation of kaempferol was induced via ER-dependent pathway. However, at 10-4 M, concentration higher than the concentrations at which the estrogenic effects of kaempferol are detected (10-5 M), kaempferol induced strong antiproliferative effect, but were unaffected by the addition of excess E2 (10-7 M) indicating that kaempferol exerts antiproliferation via ER-independent pathway. In particular, kaempferol blocked the focus formation induced by E2, which confirms that kaempferol might inhibit the malignant transformation caused by estrogens. Therefore, we suggested that kaempferol might regulate a suitable level of estrogenic activity in the body and is expected to have potential beneficial effects in preventing estrogen imbalance diseases (breast cancer, osteoporosis, cardiovascular disease and etc.).