A forced vibration method for measuring the frequency dispersion of internal friction under cyclic-tensile-compressive stress on the single axis was developed so that various mechanical tests can be conducted using a specimen having the same shape, machining, and sintering conditions. With this new method, frequency dispersions of the internal friction for tetragonal and cubic zirconia polycrystals stabilized with yttria were measured. The activation energy for the relaxation time was determined. As a result of comparison with the activation energy for the relaxation time measured by a conventional method, we conclude that the results of the internal friction measured in this study are valid. The activation energy for the ionic conductivity was determined, and compared with that for the relaxation time. The activation energies for the relaxation time in 2.6YSZ, 3YSZ, and the lower temperature peak of 10YSZ are very similar to those for the conductivity. As a result, we conclude that the internal friction peaks of 2.6YSZ and 3YSZ, and the lower temperature peak of 10YSZ are derived from rearrangement of point defects associated with oxygen ion vacancies. (C) 2003 Kluwer Academic Publishers.