Porous ZrO2 ceramics were fabricated by adding Zr(OH)4 hard agglomerates to ZrO2 powder, followed by pressureless sintering. The mechanical properties of porous ceramics sintered from pure ZrO2 powder were poor. The addition of Zr(OH)4 increased the strength and fracture toughness of the porous ZrO2 ceramics for sintered specimens containing lower porosity. However, the Young’s modulus had little change so that the strain to failure of porous ZrO2 ceramics increased with the incorporation of Zr(OH)4. Scanning electron microscopy (SEM) observations revealed that microstructures of the green compacts prepared from pure ZrO2 powder were nonuniform due to the ZrO2 soft agglomeration, which resulted in a localized nonuniform shrinkage during densification. The localized nonuniform shrinkage led to a weaker grain bonding and degraded the mechanical properties of porous ZrO2 ceramics. In this work, we found that this microstructure nonuniformity could be eliminated by the addition of Zr(OH)4, because the bimodal particle size distribution confined the formation of ZrO2 soft agglomerates due to a space constraint and an internal friction between the Zr(OH)4 hard agglomerates during compaction. As Zr(OH)4 decomposed into ZrO2 grains during heating, the Zr(OH)4 hard agglomerates disappeared before sintering occurred. The present study indicates that Zr(OH)4 hard agglomerate is a unique agent to improve the mechanical properties of porous ZrO2 ceramics.