A fracture mechanics approach was used to investigate the high strength of hot-pressed Si3N4. Room-temperature flexural strengths, fracture energies, and elastic moduli were determined for material fabricated from α- and β-phase Si3N4 powders. When the proper powder preparation technique was used, α-phase powder resulted in a high fracture energy (69,000 ergs/cm2), a high flexural strength (95,000 psi), and an elongated (fiberlike) grain morphology, whereas β-phase powder produced a low fracture energy (16,000 ergs/cm2), a relatively low strength (55,000 psi), and an equiaxed grain morphology. It was hypothesized that the high strength of Si3N4 hot-pressed from α-phase powder results from its high fracture energy, which is attributed to the elongated grains. High-strength Si3N4 has directional properties caused, in part, by the elongated grain structure, which is oriented preferentially with respect to the hot-pressing direction.