Breast cancer remains the leading cause of deaths among women in the world. It has a complex biological mechanisms and it is challenging to experimentally access the human body in vivo. In response to these issues, researchers have developed vascularized breast cancer models in PDMS microfluidic platforms as a new tool to better understand the complex mechanisms behind human breast cancer. However, there remain some challenges in recapitulating all aspects of breast cancer using current microfluidic breast cancer models. The majority of vascularized breast cancer models that have been developed as of 2018 do not consider the effects of hormone and hydrophobic anti-cancer drugs. This results from small hydrophobic molecules such as hormone and some drugs that are absorbed or adsorbed to the PDMS, which comprises hydrophobic polymer networks. In response, we develop a 3D perfusable vasculature model combined with breast cancer cells by co-culturing HUVECs and breast cancer cell lines (MCF-7 or MDA-MB-231) in an absorption-free cyclic olefin copolymer (COC) microfluidic device to analyze the effects of the major female hormone estradiol and the anti-cancer drug tamoxifen, which are both small hydrophobic molecules. First, we confirmed that estradiol stimulates the cell proliferation of ER-positive MCF-7 cells contrary to ER-negative MDA-MB-231 cells. Second, it was observed that estradiol prevented cell apoptosis of MCF-7 seeded with fibrin gel induced by treatment of tamoxifen. Finally, from our vasculature model combined with breast cancer cells in a COC microfluidic device, we presented that the treatment of estradiol with tamoxifen enhanced the cell viability of vascular networks as well as MCF-7 cells compared to tamoxifen treatment alone. These results demonstrate that it is essential to consider the effects of the hormone microenvironment and hydrophobic anti-cancer drugs in a microfluidic platform for the studies of vascularized breast cancer models.