After polycrystalline silicon nitride samples with various alpha/beta phase ratios were fabricated by uniaxial hot-pressing under vacuum, their microstructures, alpha-to-beta transformations, and grain-growth mechanisms were then characterized by electron microscopy. Room-temperature fracture toughness (K-IC) increased with increasing beta-phase content, and a value of 7.0 +/- 0.5 MPa . m(1/2) was obtained for a sample that contained 60 vol % beta phase. The increase in K-IC is the result of an increase in elongated-grain fractions with increasing beta content, which promotes energy absorption by crack deflection, as well as by grain debonding, pullout, and bridging mechanisms.