With new characterization techniques, the structures of green and sintered bodies were examined in great detail to understand the formation mechanism of potential fracture origins in alumina ceramics made by the powder compaction process. Large pores, which are found at many fracture origins, were formed in green bodies through two mechanisms. One is created from the dimple in the powder granules and the other from inadequate cohesion of the granules. Another potential cause of fracture, the large grains, developed from large particles in the green body. The concentration, especially of large pores was found to be quite high. This is clearly responsible for the moderate strength with high Weibull's modulus of the alumina ceramic examined in this study.