Microstructures of a layered peridotite ultramylonite from the Oman ophiolite are compared with that of experimentally deformed samples. Average grain sizes and grain size ratios of olivine and pyroxene from each layer are compared with respect to the fraction of pyroxene (f(px)) in the layer. Grain size of the pyroxene is almost constant among different f(px) layers, whereas olivine grain size decreases significantly with increasing f(px), both of which were characteristic features found in forsterite+enstatite aggregates after grain growth experiments (Tasaka and Hiraga, ). Furthermore, the Zener relationship (log d(ol)/d(px) versus log f(px)) found in the ultramylonite is remarkably comparable to that observed in our experiments. These observations indicate effective pinning of olivine grain growth due to the presence of pyroxene grains during the deformation of the rocks. Olivine grains in layers with f(px)0.03 do not exhibit lattice-preferred orientation (LPO), whereas the grains in layers with f(px)<0.03 exhibit LPO, indicating that deformation proceeded via diffusion- and dislocation-accommodated creep in the former and the latter layers, respectively. We simulated the evolution of grain size and viscosity in the shear zone based on our grain growth and flow laws obtained for diffusion creep of forsterite+enstatite (Tasaka and Hiraga, ; Tasaka et al., ) and successfully reproduced the observed grain sizes in the ultramylonite. We therefore conclude that the relative values of the kinetic parameters, some of which are functions of the f(px), are applicable to nature.