The coefficient of thermal expansion (CTE) of a 140‐μm‐diameter SiC monofilament was determined to be 6.5 ± 0.5 × 10−60C−1in the temperature range 25°–900°C. Heat treatment of the fibers at 1400°C for 90 min resulted in both an increase in the intensities of the SiC peaks and a reduction of their width, indicating some grain growth. Furthermore, heat treatment of the fibers for 2 h at 1600°C in vacuum reduced their CTE to 5.7 × 10−60C−1. The anomalously high CTE and its decrease upon heat treatment were attributed to the presence of a large fraction of the atoms in the boundaries between the nanometer β‐SiC grains. The thermal expansion coefficients of a series of SiC/borosilicate composites were measured as a function of fiber volume fraction in the temperature range 25°–500°C and found to