AbstractSilicon nitride (Si₃N₄) ceramics with initial β‐Si₃N₄ content from 3% to 88% were prepared by spark plasma sintering from 1450 to 1700°C. The effect of initial β‐Si₃N₄ content on the phase transition, microstructure evolution, and mechanical properties of the Si₃N₄ ceramics was studied. The existence of initial β‐Si₃N₄ accelerated the α–β phase transition. When the initial β‐Si₃N₄ content was 12% and 46%, the aspect ratio of the grains was increased. When the initial β‐Si₃N₄ content was 88%, the grains were mainly composed of short rod‐like grains. As the initial β‐Si₃N₄ content increased, the densification process was slowed down. The hardness and fracture toughness of samples reached about 18 GPa and 6 MPa·m^1/2. It is proposed that the mechanical properties of samples are connected with the densification process and β‐Si₃N₄ content.