Tantalum pentoxide (Ta 2 O 5 ) thin films were deposited on Pt(100)/Si(100) and SrRuO 3 (SRO)/Pt(100)/Si(100) substrates using an RF magnetron sputtering system at a low deposition rate $(\sim 0.4\mu \mathrm{m}/\mathrm{h})$. From the evaluated orientation and piezoelectricity of the deposited thin films, it was clarified that the Ta 2 O 5 thin films were crystallized to $\beta$-Ta 2 O 5 with piezoelectricity on both substrates. Furthermore, the crystallinity improved with increasing film thickness. The electromechanical coupling factor $k_{\mathrm{t}^{2}}$ values of the Ta 2 O 5 /Pt/Si and Ta 2 O 5 /SRO/Pt/Si samples were measured to be 0.96 and 1.44%, respectively, from the response of a high-overtone bulk acoustic resonator. The larger $k_{\mathrm{t}^{2}}$ with improved crystallinity suggests the deposition of a film mainly composed of $\beta$-Ta 2 O 5 .