Alumina ceramics were made by the powder granule compaction process and the structure of strength-limiting flaws was examined. Optically projected images obtained by transmission optical microscopy for a thin alumina specimen revealed that there were many round-shape large flaws of several tens of microns. The cross-sectional views of the large flaws observed after polishing the thinned specimen clearly showed that their structure was a crack-like pore rather than a spherical one. The fractography indicated that the fracture of the alumina ceramics originated at the crack-like pore which was a part of the corresponding large flaws located near the tensile surface of the specimen. The internal structure of the compact before firing was examined by the liquid immersion method. It was clearly shown that the large flaws resulted from non-uniform powder packing associated with granule deformation during compaction.