Ceramic matrix composites (CMCs) can be microwave-absorbent when endowing the composite constituents with proper dielectric properties. In this work, we report a new method to enhance the microwave-absorbing property of CMCs by insitu fabrication of a carbon-rich interphase at the fiber/matrix interface. This was achieved in a SiC fiber reinforced SiC matrix (SiCf/SiC) composite fabricated by precursor infiltration and pyrolysis (PIP). We found that as the PIP temperature increased from 800 to 1000 degrees C, the microwave-absorbing property of the SiCf/SiC composite was significantly enhanced at X band, which also surpassed those of the SiC fiber and monolithic SiC ceramic fabricated at the same temperature. The dominant mechanism was studied by decoupling the effect of individual SiC fibers, SiC matrix, and fiber/matrix interface. The results showed that the SiC fiber and SiC matrix were barely microwave-absorbent, due to their low dielectric losses. The microwave-absorbing mechanism was finally ascribed to the fiber/matrix interface, which was carbon-rich, containing Si and O elements. The interphase showed a conductivity that was superior to that of the fiber and the matrix, and mainly dominated the dielectric property of the overall composite. The results highlight the role of carbon-rich interphase on the microwave-absorbing property of CMCs.