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The influence of microstructure evolution on the dynamic mechanical properties, including storage modulus and internal friction, of the 2.5D SiC_f/SiC_m composites after high-temperature treatment (800 °C and 1400 °C) in the air was investigated by three-point bending vibration test. The effects of test frequency and amplitude on storage modulus and internal friction were also evaluated. The results show that as-prepared samples have maximum storage modulus and internal friction. However, the composites treated at 800 °C in the air have the minimum storage modulus due to a large number of defects produced within the composite structure, and the composites treated at 1400 °C have the minimum internal friction due to the formation of α-cristobalite in the interface between the matrix and fibers, resulting in stronger interface bonding. With regard to test conditions, the storage modulus is sensitive to amplitude but not frequency; however, the internal friction is sensitive to both frequency because of anelasticity and amplitude due to the static hysteresis.