The fracture toughness of micron (Aldrich) and submicron (Tosoh 0) ZrO2 ceramics with additions of 3.11 wt% MgO (9.25 mol% Mg-PSZ) nanopowder "as-received" (AR) and 1200 degrees C heat treated (HT) sintered at 1720 degrees C during 1 h in air was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The change in the starting (fill) density with additions of "as-received" (AR) and 1200 degrees C (HT) MgO powder, may be attributed to the bimodal particle size distribution and the presence of numerous dense agglomerates indicating that the agglomeration state was not completely destroyed during compaction process. The difference observed in the grain size indicated that the ceramics produced with submicron ZrO2 powders contained a porosity whose distribution limits the grain growth. An effective tetragonal-to-monoclinic phase transformation occurred and was reflected by the presence of intergranular and intragranular cracking in the specimens. Composites fabricated with Aldrich ZrO2 powders showed a major retention of transformable submicron tetragonal precipitates than Tosoh 0 ZrO2 samples. The variation in hardness and fracture toughness seems to be wholly dependent on the amount of the t-phase retained. The use of micron ZrO2 powder as matrix favors the increase both hardness and fracture toughness for additions of "as-received" and heat treatment MgO powder as stabilizer. (C) 2008 Elsevier B.V. All rights reserved.