Abstract: Nanocrystalline ZrO₂ (Y₂O₃) powder with different particle sizes was prepared by a boiling hydrolysis method. SiC/ diamond composite toughened with the ZrO₂ (Y₂O₃) was sintered at high temperature (1350℃) and ultra high pressure (5GPa). The phase structure and mechanical properties of the sintered material were investigated by TEM, SEM, XRD and the measurements of impact toughness and G ratio. The results showed that the amount of t ZrO₂ in the sintered material increases with boiling hydrolysis time during ZrO₂ powder preparation. 100% t ZrO₂ was obtained and the particle size distribution is homogeneous on the SiC bonded phase with the boiling hydrolysis time more than 50hr. More than 20vol% of t ZrO₂ is transformed to m ZrO₂ during the fracture process. The SiC/ diamond composite has a combination property of high impact toughness and high G ratio.