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The effects of HIPing on the defect size distribution and the mechanical properties of ceramics were examined. Specimens of Y-TZP were HIPed under several conditions. Microstructures of the specimens were observed by SEM. The defect size and number were directly measured by transmission optical microscopy. The fracture toughness was measured by SEPB method. In all specimens, larger number of defects exist in the range of smaller defect size. Total number of defects and the largest defect size were reduced by HIPing. However, even in HIPed specimens, there are a few defects having several tens micrometers long. The fracture origin appeared to change from the bulk defects to the surface ones with reduction of the total defect number and the largest defect size by HIPing.