For the pressure-assisted master sintering surface (PMSS), the sintered density during hot pressing is a unique function of the integral of a temperature function over time at a fixed pressure, irrespective of the heating path. The present research was undertaken to develop the pressure-assisted master sintering surface for silicon nitride known as potential material for high-temperature structural applications due to its excellent mechanical and thermal properties. High-purity α-Si3N4 powder (E-10 grade, Ube Industries) were mixed with 6.25% Y2O3 by alumina ball milling in ethanol for 3 hours. From the weight loss of ball mill after mixing, 1% Al2O3 was added. Consequently, SiN, 6.25% Y2O3 and 1% Al2O3 of powder mixture were prepared. After drying the resultant slurry at 100°C, the mixed powder were cold pressed at 300Mpa and made as disk of 25.4mm diameter and 2mm thickness. Then, densifications of silicon nitride were continuously recorded during heating at two different ramping rates of 5°C/min and 10°C/min up to 1800°C at fixed pressures from 7 to 34 MPa. The pressure-assisted master sintering surface of silicon nitride was successfully constructed. Using this surface, the final density can be predicted to about 1% accuracy for a fixed pressure and an arbitrary temperature-time path.