Objective To explore the role of estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) in estrogen-induced proliferation of endometrial cancer, and explore whether metformin inhibits the proliferation of endometrial cancer cells through ERα and ERβ. Methods Stable transfected Ishikawa cells were constructed by lentivirus. The effects of down-regulated ERα and ERβ on estrogen-induced Ishikawa cell proliferation were detected by methyl thiazolyl tetrazolium assay. The effects of down-regulated ERα and ERβ on estrogen-induced Ishikawa cell cycle were detected by flow cytometry. In addition, quantitative real-time polymerase chain reaction and Western blotting assays were used to detect changes in the expression of cyclinD1 and P21 involved in cell cycle regulation. The effects of down-regulated ERα and ERβ on estrogen-induced Ishikawa cell proliferation were observed by adding metformin to estrogen treatment. Results Down-regulation of ERα inhibited the proliferation and cell cycle of Ishikawa cells (P<0.05). Down-regulation of ERα also inhibited the expression of cyclinD1 and promoted the expression of P21 (P<0.05). Down-regulation of ERα counteracted the effect of estrogen-induced cell proliferation, cell cycle, and the expression changes of cyclinD1 and P21 (P<0.05). Down-regulation of ERβ promoted the proliferation and cell cycle of Ishikawa cells (P<0.05). Down-regulation of ERβ also promoted the expression of cyclinD1 and inhibited the expression of P21 (P<0.05). Down-regulation of ERβ enhanced the effect of estrogen-induced cell proliferation, cell cycle, and the expression changes of cyclinD1 and P21 (P<0.05). Metformin inhibited the proliferation of estrogen-induced Ishikawa cells (P<0.05), while in the down-regulated ERα Ishikawa cells or down-regulated ERβ Ishikawa cells, the inhibition of metformin on Ishikawa cells disappeared (P<0.05). Conclusions ERα may promote estrogen-induced proliferation of endometrial cancer cells, while ERβ may inhibit estrogen-induced proliferation of endometrial cancer cells. In addition, ERα and ERβ may also mediate the inhibitory effect of metformin on endometrial cancer cells.