Alumina bicrystals with low-angle and near-Sigma3 [0001] tilt grain boundaries were fabricated using diffusion bonding to study the dislocation structures in alumina grain boundaries. The resulting grain-boundary structures were investigated using high-resolution transmission electron microscopy, and the grain-boundary energies were analyzed using theoretical calculations. It was found that partial dislocations with Burgers; vectors of the type 1/3[10 (1) over bar0] were periodically located in the boundaries and that a stacking fault between pairs of partials was formed in such boundaries. The length of the stacking fault decreased with increased misorientation angles, which was reasonably predicted by the theoretical calculation. In the case of a near-Sigma3 grain boundary, an array of displacement shift complete dislocations with the Burgers vector of 1/3[1 (1) over bar 00] was periodically formed along the boundaries. These boundaries did not have stacking faults. The spacing between the dislocations decreased with increased deviation angle from the exact-Sigma3 boundary with the tilt angle of 60degrees.