Abstract: The rheological strengths of the major minerals in the Earth's mantle have been measured using in situ synchrotron X-ray diffraction at high pressures and high temperatures. Deconvolution of the diffraction peak widths is used to derive the yield strengths. The experiments indicate that minerals in the upper mantle and transition zone have a very different rheological character from the lower mantle minerals. The strengths of olivine, wadsleyite and ringwoodite are very sensitive to temperature, and that of olivine is weakened significantly more than for wadsleyite and ringwoodite at relatively low temperatures, being only about 1/3 wadsleyite’s strength at 873 K. In contrast, the strength of perovskite is very insensitive to temperature. The characteristics of rheological strength, i.e. stress accumulation capability and temperature dependence, may play a crucial role in defining the profile of deep earthquake occurrence as a function of depth.