Abstract: nano SiC fiber modified chopped carbon fiber (nano SiC_f-C_fd) was prepred by catalytic chemical vapor deposition using Methyltrichlorosilane (MTS) as the precursor. Afterwards, nano SiC_f-C_fd/Si₃N₄ and C_fd/Si₃N₄ composites were manufactured by two-steps process, gelcasting and atmospheric sintering. The influence of nano SiC_f-C_fd and C_fd on dielectric response and microwave absorption properties within X-band(8.2-12.4 GHz) was studied by using network analyzer. The results show that the complex dielectric constant and dielectric loss tangent (tan δ)of two composites increase with the added amount of fiber. At the same fiber content, the real part of the dielectric constant of nano SiC_f-C_fd/Si₃N₄ composite is lower than that of C_fd/Si₃N₄ composites, but tan δ increases. Additionally, the results of calculated reflectivity show that nano SiC_f-C_fd/Si₃N₄ composites have strong microwave absorption. When the content of nano SiC_f-C_fd is 2wt% and the thickness is 2.5 mm, the maximum absorption peak can reach -14.95 dB. Moreover, the bandwidth of reflectivity (less than -5 dB) is up to 3.5 GHz. Therefore, nano SiC_f modification can effectively improve the microwave absorption property of C_fd/Si₃N₄ composites.