Polycrystal elasticity of the unquenchable MgSiO3 high-pressure clinoenstatite has been measured using ultrasonic interferometry in conjunction with in-situ X-ray diffraction and imaging techniques within its stability field in a large volume multi-anvil apparatus for the first time. At 6.5 GPa, we obtained ∂KS/∂P = 5.6(3), ∂G/∂P = 1.5(1), ∂KS/∂T = -0.017(1) GPa K-1 and (∂G/∂T) = -0.015(1) GPa K-1. Our results show that the elastic velocities of HP-CEN are not only higher than those of OEN but also higher than olivine at upper mantle conditions. According to these new data, the elastic velocities for the depleted mantle composition, with ~20 vol% of HP-CEN, would result in ~0.8% and ~1.6% in difference for P and S waves, respectively, when OEN data were used in previous calculations. Newly measured results also suggest that it is possible to distinguish undepleted and depleted composition models in the upper mantle.