When pure alumina is sintered at 1620 degrees C, normal grain growth occurs with equiaxial grains and curved grain boundaries. When 100 ppm of SiO2 together with 50 ppm of CaO is added, abnormal grain growth (AGG) occurs with large grains elongated with straight grain-boundary segments in the direction of the basal planes. Some of the fine matrix grains also have straight grain boundaries, and similar to 10% of the grain boundaries of the matrix grains are faceted when observed by transmission electron microscopy (TEM), Some of these grain boundaries are expected to be singular with low-energy structures corresponding to the cusps in the polar plot of the grain-boundary energy against the inclination angle. No frozen liquid is found at the grain triple junctions and grain boundaries by TEM. When 600 ppm of MgO is added together with 100 ppm of SiO2 and 50 ppm of CaO normal growth occurs. The grain boundaries are curved when observed via optical microscopy and TEM and show that all the grain boundaries are defaceted, indicating that they become atomically rough. When sintered at 1900 degrees C after adding 150, 300, or 500 ppm of SiO2, AGG occurs with straight and faceted grain boundaries, similar to the specimens sintered at 1620 degrees C after CaO and SiO2 are added. When MgO is added together with SiO2 and sintered at 1900 degrees C, normal grain growth occurs with rough grain boundaries. High-resolution TEM observation shows no frozen liquid layer at a grain boundary. The results indicate that the occurrence of AGG in alumina with SiO2 or together with CaO is correlated with the formation of faceted and straight ton large and atomic scales) grain boundaries. It is proposed that these grain boundaries have singular ordered structures with low boundary energies and their growth by lateral step movement can cause the AGG, The addition of MgO causes grain-boundary roughening and, thus, normal grain growth. The grain boundaries in pure alumina also appear to be rough, and, hence, normal grain growth occurs.