Although microstructural evolution is critical to strain-dependent processes in Earth's mantle, flow laws for dunite have only been calibrated with low-strain experiments. Therefore, we conducted a series of high-strain torsion experiments on thin-walled cylinders of iron-rich olivine aggregates. Experiments were performed in a gas-medium apparatus at 1200°C and constant strain rate. In our experiments, each at a different strain rate, a peak stress was observed followed by significant strain weakening. We first deformed samples to high enough strain that a steady state microstructure was achieved and then conducted strain rate stepping tests to characterize the creep behavior of each sample with constant microstructure. A global fit to the data yields a stress exponent of 4.1 and a grain-size exponent of 0.73, values which agree well with those from previous small-strain experiments conducted on olivine in the dislocation-accommodated grain-boundary sliding (GBS) regime. Strong crystallographic preferred orientations provide support for GBS accommodated by movement of (010) dislocations. The observed strain weakening is not entirely explained by grain-size reduction; thus, we propose that the remaining 30% reduction in stress is related to CPO development. To incorporate microstructural evolution in a constitutive description of GBS in olivine, we (1) derive a flow law for high-strain deformation with steady state microstructure, which results in an apparent stress exponent of 5.0, and (2) present a system of evolution equations that recreate the observed strain weakening. Our results corroborate flow-law parameters and microstructural observations from low-strain experiments and provide a means for incorporating strain weakening into geodynamic simulations.