Porous Si3N4 ceramics were synthesized by pressureless sintering of green compacts prepared using slip casting of slurries containing Si3N4, 5 Wt% Y2O3 + 2 wt% Al2O3, and 0-60% organic whiskers composed of phenol-formaldehyde resin with solids loading up to 60 wt%. Rheological properties of slurries were optimized to achieve a high degree of dispersion with a high solid-volume fraction. Samples were heated at 800 degrees C in air and sintered at 1850 degrees C in a N-2 atmosphere. Porosities ranging from 0% to 45% were obtained by the whisker contents (corresponding to 0-60 vol% whisker). Samples exhibited a uniform pore distribution. Their rod-shaped pore morphology originated from burnout of whiskers, and an extremely dense Si3N4 matrix.