Since hydrogen plays an important role in dynamic processes in Earth's mantle, we conducted torsion experiments to shear strains of 0.6 to 5.0 on Fe-bearing olivine aggregates [(Mg0.5Fe0.5)(2)SiO4:Fo(50)] under hydrous conditions at T = 1200 degrees C and P=300 MPa. We deformed samples to high enough strains that a steady state microstructures were achieved, which allowed us to investigate the evolution of both the rheological and microstructural properties. The stress exponent of n approximate to 5.0 and the grain size exponent of p approximate to 0 determined by fitting the strain rate, stress, and grain size data indicate that our samples deformed by dislocation creep. Fourier transform infrared spectroscopy measurements on embedded olivine single crystals demonstrated that our samples were saturated with hydrogen during the deformation experiments. The lattice preferred orientation (LPO) of olivine changes as a function of strain due to competition among three slip systems: (010)[100], (100) [001], and (001)[100]. Observed strain weakening can be attributed to geometrical softening associated with development of LPO, which reduces the stress by similar to 1/3 from its peak value in constant strain rate experiments. The geometrical softening coefficient determined in this study is an important constraint for modeling and understanding dynamical processes in the upper mantle under hydrous conditions.