Silicon nitride (Si3N4) ceramics have a wide potential application in industrial processes, but high fracture toughness and hardness are always difficult to be handled, thus affecting the application of Si3N4 ceramics. In this paper, silicon nitride ceramics were fabricated via spark plasma sintering at 1 600 ℃ with high-entropy boride phase (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 as sintering aids. The effect of (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 on the phase assemblage, densification, microstructure and mechanical properties was investigated. Compared with (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2-free Si3N4 ceramic, the addition of only 1.0% (in volume fraction) (Hf0.2Zr0.2Ta0.2Cr0.2Ti0.2)B2 in the ceramic can increase the mass fraction of β-Si3N4 from 38% to 53% at 1 600 ℃, showing a bimodal microstructure. As a consequence, the fracture toughness of Si3N4-based ceramic increases from (5.4±0.3) MPa·m1/2 to (6.9±0.2) MPa·m1/2, and the hardness maintains (20.2±0.2) GPa. The relative density of Si3N4-based ceramics decreases and the phase transformation enhances with increasing the high-entropy boride fraction to 2.5% and 5.0%, thus leading to a decrease in the hardness.