Tests were performed on Al₂O₃-Y₂O₃ based compounds, which are the most widely used sintering additive for Si₃N₄, with the objectives of evaluating the high-temperature viscosity characteristics of Al-Y-Si-O based glass and clarifying the relationship between high-temperature viscosity and the high-temperature strength of Si₃N₄. The viscosity of the glass at 1600°C was found to be about 8-50P, while the activation energy for high-temperature viscosity was about 9-37 kJ/mol. Properties of glass such as viscosity are governed by the composition of the glass; for compositions within the glass forming region, it is thought that the network structure, hardens and viscosity becomes high in compositions having a substantial amount of SiO₂, or in compositions where the ratio of Y₂O₃/Al₂O₃ (close to the eutectic composition) is 1.0. For compositions outside the glass forming region, the influence of the precipitation of the crystal phase is considered to become more prominent. The room temperature strength of Si₃N₄ is not governed by the properties of the glass phase, but by micro structural factors such as the residual amount of pores. However, the properties of the glass phase in the sintering process are considered to determine structural factors such as the density of sintered bodies and the morphology of Si₃N₄ grains.The high-temperature strength of Si₃N₄ is dependent on the high temperature viscosity characteristics of the glass phase, and this was seen to impart high high-temperature strength to Si₃N₄ for which glass with high high-temperature viscosity would exist at the grain boundaries. For Si₃N₄ with similar viscosity characteristics at high temperatures, high-temperature strength is dependent on the size of the Si₃N₄ grains, seen to result in superior high-temperature strength for Si₃N₄ with large Si₃N₄ grains. Correlation between the rate of change in 1300°C strength and the activation energy of viscosity for Si₃N₄ was observed when evaluated in high temperature strength tests conducted at slow cross-head speeds of 0.1mm/min to 0.5 mm/min, and it was found that the rate of change in the 1300°C strength tended to become smaller for Si₃N₄ compositions with lower activation energy.