Unique ZrO2 crystals within pores were prepared by a sol-gel-hydrothermal method and characterized by XRD, TG/DTA, TEM, SEM, and N-2 adsorption-desorption isotherms. Results show that the dried sol-gel was converted into monoclinic zirconia crystals after calcined above 550 degrees C. The crystal sizes of the zirconia single crystal were in the range of 20-70 nm. A number of irregular pores were embedded within the zirconia crystals. This unique structure was obtained due to the combination of sol-gel and hydrothermal treatments. (C) 2009 The Society of Powder Technology Japan. Published by Elsevier BV and The Society of Powder Technology Japan. All rights reserved.