The mechanical properties of an ultrahigh temperature ceramic (UHTC) made up of ZrB 2 , SiC and graphite flake (ZrB 2 -SiC-G) are explored by ultrahigh-temperature uniaxial tension, combined with in-situ monitoring of the fracture behavior. The results suggest an inconsistent property degradation of ZrB 2 -SiC-G at 1200–2000 °C. Significant degradation in tensile modulus is observed at 1400 °C, which differs from the strength degradation that mainly occurs at 1800 °C. Moreover, ZrB 2 -SiC-G is elastically fractured at 1200 °C, 1400 °C and 1600 °C, which is interrupted at 1800 °C and 2000 °C. At the temperature range, severe plasticity takes place, which reduces the bearing capacity of this material. The brittle-to-ductile transition is well ascertained by in-situ observation of the fracture behavior at 1200–2000 °C. Finally, the constitutive relation of modulus and strength is built by coupling the effect of temperature. It can be integrated in finite element models and is useful for structural design of UHTC based thermal protection systems of hypersonic vehicles. [ABSTRACT FROM AUTHOR]