The mechanics of olivine deformation play a key role in long-term planetary processes, including  the response of the lithosphere to tectonic loading or the response of the solid Earth to tidal forces,  and in short-term processes, such as post-seismic creep within the upper mantle. Previous studies  have emphasized the importance of grain-size effects in the deformation of olivine. Most of our  understanding of the role of grain boundaries in the deformation of olivine is inferred from comparison  of experiments on single crystals to experiments on polycrystalline samples, as there are no direct  studies of the mechanical properties of individual grain boundaries in olivine. In this study, we use  high-precision mechanical testing of synthetic forsterite bicrystals with well characterized interfaces  to directly observe and quantify the mechanical properties of olivine grain boundaries. We conduct  in-situ micropillar compression tests at high-temperature (700• C) on bicrystals containing low-angle (4• tilt about [100] on (014)) and high-angle (60• tilt about [100] on (011)) boundaries. During  the in-situ tests, we observe differences in deformation style between the pillars containing the  grain boundary and the pillars in the crystal interior. In the pillars containing the grain boundary,  the interface is oriented at ∼ 45• to the loading direction to promote shear. In-situ observations  and analysis of the mechanical data indicate that pillars containing the grain boundary consistently  support elastic loading to higher stresses than the pillars without a grain boundary. Moreover, the  pillars without the grain boundary sustain larger plastic strain. Post-deformation microstructural  characterization confirms that under the conditions of these deformation experiments, sliding did not occur along the grain boundary. These observations support the hypothesis that grain boundaries are stronger relative to the crystal interior at these conditions.