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Porous Si3N4 ceramics were fabricated by liquid-phase sintering with a Yb2O3 sintering additive, and the microstructure and mechanical properties of the ceramics were investigated, as a function of porosity. Low densification was achieved using a lower Yb2O3 additive content. Fibrous β-Si3N4 grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. A high porosity, ∼40–60%, with β-Si3N4 grain development, was obtained by adjusting the additive content. Superior mechanical properties, as well as strain tolerance, were obtained for porous ceramics with a microstructure of fine, fibrous grains of a bimodal size distribution. [ABSTRACT FROM AUTHOR]