Strong, tough, high temperature ceramic matrix composites are currently being developed for application in advanced heat engines. These new materials require new nondestructive inspection and material characterization techniques to insure the final integrity, as well as reduce the time require for development. One of the most promising of these new materials is SiC fiber-reinforced silicon nitride ceramic matrix composite (SiCf/ Si3N4). The high temperature thermal and mechanical performance of ceramic matrix composites is strongly dependent on the thermal diffusivity and interfacial bond strength of the material. Previous work has shown a interaction between the thermal diffusivity and the fracture toughness of SiCf/Si3N4. A thermal imaging technique has been developed to provide rapid large area measurements of the thermal diffusivity perpendicular to the fiber direction in these composites. Results are presented for a series of SiCf/Si3N4 (reaction bonded silicon nitride) composite samples heat-treated under various conditions. These results are compared with previous experimental and analytical work in this field.