Ceramic powder compaction by pressing requires weak-agglomerated powders to obtain high dense compacts and consequently dense sintered ceramic body. In this work powders with different agglomerated and/or aggregated size distributions was compacted by uniaxial and isostatic pressing. The powder and compacts were characterized by mercury and nitrogen porosimetry, BET and SEM. Analysis of X-rays diffraction peaks was used to determine the particle size (diameter). The Scherrer method was used to obtain the estimated average nanoparticle size. The results showed that all aggregated and weak agglomerated powders were composed by nanoparticles. This study indicated that the nanoparticle aggregates and weak agglomerates were destroyed during the pressing stage conducting to a increase of the packing degree of the powder particles.