We have developed a new method for estimating experimentally the net energy applied to powders in a particle-compounding process with a high-speed elliptical-rotor-type powder mixer (HEM). Spherical copper particles, of which the stress-strain relation is known, are used as the standard material to obtain the net applied energy. Simplified models for the changes of the size distribution and shape of copper particles with plastic deformation have been proposed. The amount of deformation of a copper particle is determined from the diameters of projected images of particles before and after deformation. The net energy applied to the copper particles is estimated as the total of the compressive energy of individual particle. The net applied energy in single compression by the elliptical rotor is proportional to the square of the rotational speed of the rotor; the energy in single compression depends on the kinetic energy of rotational motion of the rotor. The method for estimating the net applied energy is effective in the analysis of the energy transmission from a media mill to powders and in the determination of optimal operating condition in a particle-compounding process.