Symmetrical Sigma7 tilt grain boundaries of alumina (Al2O3) were studied using bicrystals. Three types of Sigma7 boundaries were successfully fabricated, that is, rhombohedral twin (Sigma7{1 (1) over bar 02}) and two types of [0001] symmetrical tilt grain boundaries with grain-boundary planes {4 (5) over bar 10} and {2 (3) over bar 10} (Sigma7{4 (5) over bar 10} and Sigma7{2 (3) over bar 10}). Their atomic structures and grain-boundary energies were investigated using high-resolution transmission electron microscopy (HRTEM) and a thermal grooving technique, respectively. HRTEM observations showed that the Sigma7{1 (1) over bar 02} boundary had a completely symmetrical atomic arrangement with respect to the grain-boundary plane. In contrast, Sigma7{2 (3) over bar 10} and Sigma7{4 (5) over bar 10} boundaries exhibited asymmetrical atomic structures, which were confirmed by analyzing the atomic configurations using static lattice calculations. Thermal grooving experiments showed that the grain-boundary energies strongly depended on the properties of the grain-boundary planes.