Using in situ strength measurements at pressures up to 10 GPa and at room temperature, 400, 600, and 700 degrees C, we examined rheological properties of olivine, orthopyroxene, and chromian-spinel contained in a mantle-derived xenolith. Mineral strengths were estimated using widths of X-ray diffraction peaks as a function of pressure, temperature, and time. Differential stresses of all minerals increase with increasing pressure, but they decrease with increasing temperature because of elastic strain on compression and stress relaxation during heating. During compression at room temperature, all minerals deform plastically at differential stress of 4-6 GPa. During subsequent heating, thermally induced yielding is observed in olivine at 600 degrees C. Neither orthopyroxene nor spinel shows complete stress relaxation, but both retain some stress even at 700 degrees C. The strength of the minerals decreases in the order of chromian-spinel approximate to orthopyroxene > olivine for these conditions. This order of strength is consistent with the residual pressure of fluid inclusions in mantle xenoliths.