ZrB2-20 vol.%SiC ultrahigh temperature ceramic containing 5 vol.% carbon black of nanometer size (ZSC) was prepared by hot-pressing at 1900 degrees C. The effect of carbon black addition on the microstructure and mechanical properties was investigated. Compared to the ZrB2-SiC ceramics, the fracture toughness was significantly improved whereas flexural strength and hardness of the ZSC ceramic were slightly degraded. The fracture toughness as high as 6.6 +/- 0.5 MPa m(1/2) was much higher than that of monolithic ZrB2 (2.3-3.5 MPa m(1/2)) and ZrB2-SiC ceramics (4.0-4.5 MPa m(1/2)). The toughening mechanisms were the crack deflection, bridging and branching. Furthermore, the effect of carbon black addition on the thermal shock resistance was systematically investigated and discussed. (C) 2010 Elsevier B.V. All rights reserved.