Mediator is the most important coregulator in eukaryotic transcription. It conveys the signals received from transcriptional activators on enhancer sequences to transcription machinery on promoter regions. Although Mediator itself and MED1 subunit have been implicated in the activation of estrogen receptor alpha (ERα)-mediated gene expressions and breast cancer cell proliferations, the mechanism by which Mediator interacts and brings ligand-bound ERα on its cognate element site to promoter and translates their interactions to gene activation is still unknown. We aimed to identify the interaction between ERα and Mediator to clarify this mechanism. We recombinantly generated active human core Mediator complex along with other subunits of Mediator. We interestingly saw that MEDX showed a potential interaction. In addition to characterizing Mediator- ERα interaction, we also checked the role of the Mediator Complex in acquired tamoxifen resistance. We performed immobilized template recruitment assay to characterize the alterations in recruitment of Mediator subunits and RNA Pol II to 2XERE-TATA promoter in wild type and tamoxifen resistant MCF-7 cell lines. We tried to clarify how changes in pre-initiation complex formation causes resistance to tamoxifen in ERα-positive breast cancer patients. The increase in MEDZ subunit recruitment in resistant cells was striking and promising for further experiments. We hope that our findings will help to elucidate the detailed mechanism of ERα-dependent transcription and design alternative therapeutics.