【Abstract】 Objective To study the effects of different levels estradiol on inhibitory of tamoxifen on human mammary cancer cells(ER+) in vitro. Methods Estrogen receptor (ER) positive MCF7 human breast cancer cell line was treated by the same level of tamoxifen and different levels of estradiol in vitro. Cell proliferation was evaluated by MTT assay. Results E2 at concentrations between 1 × 10-12 mol/L to 1× 10-7 mol / L significantly stimulated the growth of MCF 7 . TAM (10 μmol/ L) inhabited the growth of MCF7 significantly. E2 at different levels may influence inhibitory of tamoxifen on MCF7 cell lines. E2 (1×10-8 mol/L) makes inhibitory of tamoxifen on MCF7 cell lines valueless.Conclusion E2 is the risk factor of breast cancer, and the concentration around breast cancer cells may influence the effects of TAM.