c-ZrO2/SiO2 composites with various amounts of SiO2 additions were produced by a colloidal processing and pressureless sintering at 1400 °C. The influence of the microstructure and SiO2 content on the grain size, flexural strength and fracture toughness of the composites were determined. The results showed that SiO2 had very limited solubility of 0.3 wt% in c-ZrO2 matrix. This suggests that only small part of the SiO2 dissolved in the c-ZrO2 matrix and the rest of SiO2 segregated at grain boundaries and multiple junctions as amorphous glassy phase. This glassy phase formed rounded grains and prevented the grain growth by minimizing grain boundary energy and mobility. c-ZrO2/SiO2 composites having different SiO2 contents showed high flexural strength and fracture toughness. With the increase of SiO2 content from 0 to 10 wt%, the grain size, flexural strength and fracture toughness changed from 5.7 to 0.5 μm, 275 to 292 MPa and 1.5 to 3.64 MPa m1/2, respectively. The increase in fracture toughness and flexural strength could be attributed to smaller grain size and crack deflection. © 2007 Elsevier Ltd and Techna Group S.r.l.