Submicron-sized α-Si₃N₄ and nanosized amorphous Si₃N₄ powders were sintered by spark plasma sintering at 1700°C for holding times of 60 to 120 s at heating rates of 1.7 to 6.1°C/s with Y₂O₃ and Al₂O₃ sintering additives. The nanostructure control of liquid-phase sintered Si₃N₄ ceramics was achieved by selecting optimum heating schedules such as temperature, holding time and heating rate. Fully dense nanostructured β-Si₃N₄ ceramics with grain size of 150 nm were prepared using nanosized amorphous Si₃N₄ starting powder. The transformation to β-phase in Si₃N₄ ceramics obtained from the nanosized amorphous Si₃N₄ powder was more greatly accelerated than that from submicron-sized α-Si₃N₄ powder.