The Vickers hardness of dense Si3N4 ceramics of the Si-Ce-Al-O-N system was investigated from room temperature to 1200 degrees C. A sloppy decrease of hot hardness occurred above 850-900 degrees C. A compensation law was observed between the pre-exponential factors of the hot-hardness dependence on the reciprocal of absolute temperature, and the values of the activation energy of hardness. This relationship shows that the low-temperature deformation mechanism, such as microplasticity, is competing in parallel with a grain-boundary diffusion-controlled creep process in the high-temperature range.