Abstract Flaw generation and fracture behavior in Nicalon fiber-reinforced SiC matrix composites are influenced by high temperatures. We evaluated the effects by measuring strength at several temperatures ranging from 800 to 1300 °C, and by microstructural characterization. While composite strength increased from ≈ 400 MPa at room temperature to ≈ 532 MPa at 800 °C, it decreased to ≈ 270 MPa at 1300 °C. Such degradation of composite strength at high temperature is believed to be related to fiber degradation. In-situ fiber strength in composites tested at room and elevated temperatures was estimated by fractographic techniques. The in-situ strength of fibers in composites was significantly lower than that of as-received fibers. This decrease is believed to be related to flaws that develop during processing and to exposure to elevated-temperature service environments.