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Grain boundary sliding and migration often accompany high temperature deformation in alumina, however the studies of the sliding and migration behavior of individual boundaries in bicrystals during deformation are extremely limited.In this study a single grain boundary in an alumina bicrystal was deformed at high temperature (1450°C) at a constant strain rate (2 × 10−6/s). Evolution of the grain boundary structure during deformation was monitored using optical microscopy. Movement of micron-sized facets inclined to the grain boundary plane led to the facet coarsening and produced macroscopically “curved” regions along the grain boundary.HRTEM studies reveal nanoscale analogues to such “curved” segments, which appear to contain nanoscale facets. Rumpling of the grain boundary during early stages of deformation may assist the nucleation of facets inclined to the original grain boundary plane. Macroscopic facets may develop by either growth and coarsening of atomic-height irregularities initially in the grain boundary or from atomic-height ledges formed during deformation.