The hardness and fracture toughness of a model two-phase composite consisting of crystalline Si3N4 particles in a matrix of amorphous Si3N4 are examined. The composite is created by heat treating high purity, partially amorphous CVD Si3N4 in N2 for various times and temperatures in order to induce crystallization of the a phase. Microindentation tests at temperatures up to 1200 °C are conducted to evaluate the high temperature hardness and fracture toughness. The role of the microstructure is examined using optical and transmission electron microscopy. Finally, the relationship between the microstructure and the mechanical properties is discussed.