The grain morphology, microstructure, mechanical properties and fracture behavior of hot-pressed silicon nitride containing two different sizes of TiN particles were investigated in this study. No interfacial interactions were noticeable between TiN and Si 3 N 4 up to a temperature of 1850 °C. The average aspect ratio and grain thickness of β-Si 3 N 4 grains decreased slightly with the addition of TiN particles. The amplitude and frequency of the propagating path, and the crack deflection angles, increased with the content of large TiN particles. The toughening mechanisms in TiN/Si 3 N 4 were attributed to the crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the β-Si 3 N 4 matrix. The fracture toughness of Si 3 N 4 was enhanced by 35% with the addition of 20% large TiN particles, hot-pressed at 1850 °C.