The effect of mechanical treatment of monoclinic zirconia in a high-energy centrifugal planetary ball mill and in a vibrating ball mill, with much less mechanical impact, on its crystallographic characteristics and texture was investigated as a function of the applied mechanical load and the treatment medium. Mechanical treatment in a high-energy planetary ball mill was shown to result in particle desintegration into small crystallites of less than 20 run in size, accumulation of strains, and fast solid phase transition to metastable nano-structured tetragonal form. The rate of the phase transition depended on the treatment medium. The transition completed during 15 min with dry-powdered ZrO2. The presence of water retarded the phase transition. Mechanical treatment in a vibrating mill resulted in particle desintegration into crystallites of 38-42 nm in size, accumulation of strains, and gradual amorphization of the crystalline structure; however, the transformation to the tetragonal form did not occur. The factors favored the solid phase polymorphic transition of a stable form of ZrO2 into metastable one are discussed. A high-temperature and a high pressure, which are developed in local sites of the solid under pulse mechanical impact of high-energy, provide thermodynamically favorable conditions and kinetic acceleration of the phase transition. The effect of the metastable form of ZrO2 stabilization originates from the small size of the crystallites as a result of particle desintegration. (c) 2005 Elsevier B.V. All rights reserved.