Short carbon fibers (Csf) reinforced alumina composites were prepared by a hot-pressing sintering process. The influences of Csf content and fiber length on mechanical and dielectric properties of the composites in the Ku-band (12.4–18 GHz) were evaluated. The flexure strength of the ceramic increased with the Csf incorporation and the maximum value of 475 MPa was obtained by the addition of 0.3 wt% Csf with the length being 2 mm. The electronic polarization relaxation is contributed to the variation of the real part of the permittivity (ϵ′) with increasing Csf content and fiber length. The associated polarization relaxation and conductivity of the composites are responsible for the variation of the imaginary part (ϵ″). When 0.1 wt% Csf with length of 4 mm was incorporated, the reflection loss below −6 dB can be obtained in the Ku-band with the coating thickness being 1.2 mm. The results indicate that Csf/Al2O3 composite could be a potential microwave absorbing material used in the Ku-band.