An attempt was made to develop an engineering ceramic plastically deformable at high temperatures with low flow stress at high strain rates, and without strain hardening. Dense ceramic preforms were fabricated by pressureless sintering Si3N4 + SiO2 mixed powders with an addition of MgAl2O4 at 1500 °C. A transient liquid, which occurs during the reaction sintering of Si2N2O, was utilized for subsequent net-shape forming. The ceramic (6Φ × 6 mm column) was deformed without any cracks and cavities in compression tests at high strain rates (10−2−10−3:s−1) at 1500 °C, but this was not achieved in a test at lower strain rates for a long time, because of the growth of elongated Si2N2O grains during the test. Potassium fluoride (KF) was used as a dopant for retardation of nucleation of Si2N2O during sintering and hot-working. The KF-doped preforms were successfully plastically deformed even in the test for a long time.