Tensile tests on hot-pressed silicon nitride containing 3 weight% alumina and 5 weight% yttria were conducted at elevated temperatures (l300°C.and above). Yielding phenomena somewhat similar to those of, for example, mLld steels were observed in the stress/displacement diagrams. Although these phenomena were greatly affected by the displacement rate as well as the temperature, the ultimate strength tended to depend only on the temperature. ,the yielding phen,omena are attributed to cavitation caused by the reductwn m viscosity ofthe gram boundary glassy phase.