We aim to investigate the anticancer effects and mechanisms of icaritin against breast cancer. Both estrogen receptor (ER) positive breast cancer cells MCF-7 and ER-negative MDA-MB-231 cells were employed. We examined the effects of icaritin on the proliferation and migration by wound healing assay and transwell assay. Cell apoptosis and cell cycle of MCF-7 and MDA-MB-231 cells were analyzed using Flow cytometry. Cell autophagy of MCF-7 and MDA-MB-231 cells was assessed by western blotting, acridine orange staining and confocal microscopy. We also detected the expression of apoptosis related genes by western blotting. In addition, an autophagy inhibitor was used to investigate whether cytoprotective autophagy was induced. Meanwhile, an ER inhibitor was utilized to explore whether ER was involved in autophagy. Icaritin inhibited the proliferation and migration, and induced cell cycle arrest of both MDA-MB-231 and MCF7 cells. Icaritin significantly induced apoptosis of MDA-MB-231 cells by activating caspase-3. And icaritin stimulated autophagy in MCF-7 cells, as evidenced by increased LC3II/LC3I, enhanced p62 degradation, the accumulation of endogenous LC3 puncta formation, and the increased autophagy flux. Icaritin induced autophagy through upregulating the phosphorylation of AMPK and ULK1. Chloroquine, an autophagy inhibitor, increased icaritin-induced apoptosis and proliferation inhibition of MCF-7 cells. Meanwhile, tamoxifen, an ER inhibitor, reversed icaritin-induced autophagy and proliferation inhibition of MCF-7 cells. Our study demonstrated that the antitumor effects of icaritin against breast cancer are related with ER, which suggested that the status of ER should be considered in clinical application of icaritin. Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.