An origin of the strength variation of silicon nitride ceramics with cold isostatic pressure (CIP) condition was investigated for a powder granule compaction process. Direct observation methods were successfully applied to examine the micro structures of granules, green bodies and their sintered specimens. These methods revealed a significant variation in pore structure with CEP condition for both green and sintered bodies. The strength variation with CIP condition was attributed to the different sizes and concentration of large pores el in sintered bodies, which were developed or persisted during liquid phase sintering. The large pores were formed from the dimple in the spray-dried granule and the boundaries of the granule traces in green compacts.