Single-phase solid solution monoclinic structure high-entropy tantalates (Nd_1/6Sm_1/6Eu_1/6Gd_1/6Dy_1/6Ho_1/6) TaO₄(6RETaO₄), (Nd_1/5Sm_1/5Eu_1/5Gd_1/5Dy_1/5)TaO₄(5RETaO₄), (Nd_1/4Sm_1/4Eu_1/4Gd_1/4)TaO₄(4RETaO₄) were prepared by solid state method. STEM-EDS result shows the rare earth elements are uniformly distributed without segregation. The ferroelastic domain observed through SEM derived from the second ferroelastic phase transition. The thermal expansion experiment suggests the good thermal stability below 1200 ℃, where the thermal expansion coefficient of 6RETaO₄ reaches 9.25×10^-6K^-1 at 1200 ℃. Due to the increase of phonon scattering derived from high-entropy effect, RETaO₄ ceramics exhibit lower intrinsic thermal conductivity (2.98-1.23 W·m^-1·K^-1, 100-1000 ℃) and enhanced mechanical properties (6RETaO₄, (9.97±2.2) GPa), which indicates that it is a potential material for thermal barrier coatings.