Background: Nowadays, the encapsulation of cytotoxic chemotherapeutic agents is attracting interest as a method for drug delivery. We hypothesized that the efficiency of helenalin might be maximized by encapsulation in ${eta}$-cyclodextrin nanoparticles. Helenalin, with a hydrophobic structure obtained from flowers of Arnica chamissonis and Arnica Montana, has anti-cancer and anti-inflammatory activity but low water solubility and bioavailability. ${eta}$-Cyclodextrin (${eta}$-CD) is a cyclic oligosaccharide comprising seven D-glucopyranoside units, linked through 1,4-glycosidic bonds. Materials and Methods: To test our hypothesis, we prepared ${eta}$-cyclodextrin-helenalin complexes to determine their inhibitory effects on telomerase gene expression by real-time polymerase chain reaction (q-PCR) and cytotoxic effects by colorimetric cell viability (MTT) assay. Results: MTT assay showed that not only ${eta}$-cyclodextrin has no cytotoxic effect on its own but also it demonstrated that ${eta}$-cyclodextrin-helenalin complexes inhibited the growth of the T47D breast cancer cell line in a time and dose-dependent manner. Our q-PCR results showed that the expression of telomerase gene was effectively reduced as the concentration of ${eta}$-cyclodextrin-helenalin complexes increased. Conclusions: ${eta}$-Cyclodextrin-helenalin complexes exerted cytotoxic effects on T47D cells through down-regulation of telomerase expression and by enhancing Helenalin uptake by cells. Therefore, ${eta}$-cyclodextrin could be superior carrier for this kind of hydrophobic agent.