he effect of ball-milling on the properties of ultrafine ZrO2 (Y2O3) powders prepared by coprecipitation under different conditions is investigated. It is shown by the expertimental results that the agglomerate size of as-calcinated powders can be effectively decreased by ball-milling but the primary particle size is hardly affected. The surface area of seriously agglomerated powders can be markedly increased by ball-milling but that of softly agglomerated powders is not evidently increased. The crystal size of as-calcinated powders becomes much finer after ball-milling because transformation of the powder from tetragonal structure to monoclinic structure occurs during ball-milling. It is also shown that ball-milling can be used to improve the sinterability of seriously agglomerated powders.