Intergranular sliding and diffusive mechanisms behind the deformation behavior of a commercially available lutetium-doped silicon nitride were investigated and discussed. A method of locating and separating phenomena critical for mechanical relaxation at elevated temperatures was applied; the method was based on low-frequency forced-vibration damping measurements. The potentiality of lutetium addition for improving the deformation resistance of silicon nitride was clearly reflected in the high-temperature damping behavior of the investigated polycrystal. Softening of intergranular lutetium silicate phases located at multigrain junctions, which resulted in a grain-boundary sliding peak, occurred at remarkably high temperatures (> 1725 K). This phenomenon, partly overlapping diffusional flow, was followed by further damping relaxation with the melting of the lutetium silicates. Subsequent grain growth was also detected at temperatures > 2100 K. Torsional creep results, collected up to 2100 K, consistently proved the presence of a "locking" effect by lutetium silicates with the sliding of silicon nitride grain boundaries below 1873 K.