Background: HSP90 may be overexpressed in cancer cells which are greatly dependent on Hsp90 function. Geldanamycin derivative 17 allylamino-17-demethoxygeldanamycin (17-AAG) inhibits the function and expression of HSP90. 17-AAG has poor water-solubility which is a potential problem for clinical practice. In this study for improving the stability and solubility of molecules in drug delivery systems we used a ${eta}$-cyclodextrin-17AAG complex. Materials and Methods: To assess cytotoxic effects of ${eta}$-cyclodextrin-17AAG complexes and free 17AAG, colorimetric cell viability (MTT) assays were performed. Cells were treated with equal concentrations of ${eta}$-cyclodextrin- 17AAG complex and free 17AAG and Hsp90 gene expression levels in the two groups was compared by real-time PCR. Results: MTT assay confirmed that ${eta}$-cyclodextrin- 17AAG complex enhanced 17AAG cytotoxicity and drug delivery in T47D breast cancer cells. The level of Hsp90 gene expression in cells treated with ${eta}$-cyclodextrin- 17AAG complex was lower than that of cells treated with free 17AAG (P=0.001). Conclusions: The results demonstrated that ${eta}$-cyclodextrin- 17AAG complexes are more effective than free 17AAG in down-regulating HSP90 expression due to enhanced ${eta}$-cyclodextrin-17AAG uptake by cells. Therefore, ${eta}$-cyclodextrin could be superior carrier for this kind of hydrophobic agent.