The microstructural and mechanical properties of Si3N4-based composites with different amounts of TiC0.5N0.5, fabricated by hot-pressing at 1650degreesC for 1 hour, are investigated and evaluated. The hardness of the micron-sized TiCN powder based composites increased with increasing TiCN content from 20 up to 40 vol. %, whereas the bending strength decreased. The fracture toughness reaches a maximum at 30 vol. % TiCN and exhibits a strong anisotropy with respect to the hot-pressing direction. The 30 vol % TiN composite, used as a reference material, has a lower hardness and strength but a significantly higher and more isotropic toughness. The influence of the TiCN particle size on the composite properties was studied using a nano-sized TiCN starting powder. The hardness of the composite was significantly increased using the finer powder, but the toughness was lower. The alpha/beta Si3N4 ratio was measured by X-ray diffraction revealing that the alpha-beta conversion during hot-pressing is strongly reduced with increasing TiCN content and decreasing TiCN particle size.