High temperature plasticity of fine-grained ceramics (ZrO2, Al2O3, etc) is usually associated with a grain boundary sliding process. The aim of the present research is then to improve the high-temperature mechanical strength of polycrystalline zirconia (3Y-TZP) through the insertion of multiwalled carbon nanotubes (CNTs) or silicon carbide whiskers (SiCw), which are susceptible to pin the grain boundaries. The effect of these nano-sized particles on grain boundary sliding has been studied by mechanical spectroscopy.