The room‐temperature mechanical properties of a SiC‐fiberreinforced reaction‐bonded silicon nitride composite were measured after 100 h treatment in nitrogen and oxygen environments to 1400°C. The composite heat‐treated in nitrogen to 1400°C showed no appreciable loss in properties. In contrast, composites heat‐treated in oxygen from 600° to 1000°C retained ∼65% and 35% of the matrix fracture and ultimate strength, respectively, of the as‐fabricated composites, and those heat‐treated from 1200° to 1400°C retained greater than 90% and 65% of the matrix fracture and ultimate strength, respectively, of the as‐fabricated composites. For all nitrogen and oxygen treatments, the composite displayed strain capability beyond the matrix fracture strength. Oxidation of the fiber surface coating, which caused degradation of bond between the fiber and matrix and reduction in fiber strength, appears to be the dominant mechanism for property degradation of the composites oxidized from 600° to 1000°C. Formation of a protective silica coating at external surfaces of the composites at and above 1200°C reduced oxidation of the fiber coating and hence degrading effects of oxi