Well-dispersed slurries of submicron-sized alumina powders were pressure-infiltrated in 3D preforms of mullite fibers and the effects of the particle size and infiltration pressure on the particle packing characteristics were investigated. Infiltration without pressure showed that the packing density increased as the particle size decreased due to the reduction of the friction between the particles and the fibers. The infiltrated preforms contained large pores in the large voids between the fiber tows and small pores in the narrow voids between the individual fibers. Pressure infiltration resulted in a packing density of 77% regardless of the particle size or the infiltration pressure(210 ~ 620 kPa). Pressure infiltration shortened the infiltration time and eliminated the large pores in preforms infiltrated with the slurries of smaller particles. The slurry pressure-infiltration process is thus an efficient method for the packing of matrix materials in various preforms.