Based on thermodynamic analysis, highly dense (TiB 2 +TiC)/Ti 3 SiC 2 composite ceramics with different TiB 2 volume contents were in situ fabricated in situ by hot-pressing at 1500°C. Laminar Ti 3 SiC 2 grains, columnar TiB 2 grains and equiaxed TiC grains were clearly identified from microstructural observation; grain boundaries were clean. The increase of TiB 2 volume content significantly restrains the grain growth of the Ti 3 SiC 2 matrix. As the content of TiB 2 increases from 5vol.% to 20vol.%, the bending strength and fracture toughness of the composites both increase and then decrease, whereas the Vickers hardness increases linearly from 6.13GPa to 11.5GPa. The composite with 10vol.% TiB 2 shows the optimized microstructure and optimal mechanical properties: 700MPa for bending strength; 9.55MPam 1/2 for fracture toughness. These are attributed to the synergistic action of strengthening and toughening mechanisms such as particulate reinforcement, crack deflection, grain's pull-out and fine-grain toughening, caused by the columnar TiB 2 grains and equiaxed TiC grains.