Among the various ceramic substrate materials, Sisub.3Nsub.4 ceramics have demonstrated high thermal conductivity, good thermal shock resistance, and excellent corrosion resistance. As a result, they are well-suited for semiconductor substrates in high-power and harsh conditions encountered in automobiles, high-speed rail, aerospace, and wind power. In this work, Sisub.3Nsub.4 ceramics with various ratios of α-Sisub.3Nsub.4 and β-Sisub.3Nsub.4 in raw powder form were prepared by spark plasma sintering (SPS) at 1650 °C for 30 min under 30 MPa. When the content of β-Sisub.3Nsub.4 was lower than 20%, with the increase in β-Sisub.3Nsub.4 content, the ceramic grain size changed gradually from 1.5 μm to 1 μm and finally resulted in 2 μm mixed grains. However, As the content of β-Sisub.3Nsub.4 seed crystal increased from 20% to 50%, with the increase in β-Sisub.3Nsub.4 content, the ceramic grain size changed gradually from 1 μm and 2 μm to 1.5 μm. Therefore, when the content of β-Sisub.3Nsub.4 in the raw powder is 20%, the sintered ceramics exhibited a double-peak structure distribution and the best overall performance with a density of 97.5%, fracture toughness of 12.1 MPa·msup.1/2, and a Vickers hardness of 14.5 GPa. The results of this study are expected to provide a new way of studying the fracture toughness of silicon nitride ceramic substrates.