The microstructure, mechanical, and thermal properties of in situ hot-pressed 30 vol% (ZrB2+ZrC)/Zr-3[Al(Si)](4)C-6 composite have been investigated and compared with monolithic Zr-3[Al(Si)](4)C-6 ceramic. The composite is composed of ZrB2 and ZrC grains embedded in a Zr-3[Al(Si)](4)C-6 matrix. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 415 GPa), strength (bending strength of 621 MPa), and toughness (fracture toughness of 7.37 MPa center dot m(1/2)) compared with monolithic Zr-3[Al(Si)](4)C-6. The composite retains high modulus of 357 GPa at 1430 degrees C (86% of that at ambient temperature) due to clean grain boundaries with no glassy phase. In addition, the composite exhibits higher specific heat capacity and thermal conductivity but slightly lower coefficient of thermal expansion compared with monolithic Zr-3[Al(Si)](4)C-6. The calculation of the thermal stress fracture resistance parameter (R) predicts a much improved thermal shock resistance of the composite. Based on these results, (ZrB2+ZrC)/Zr-3[Al(Si)](4)C-6 composites show promising potential for high-temperature and ultra high-temperature applications.