In the frequency range of 2-18 GHz, the microwave absorbing properties of a carbon black/silicone rubber blend were investigated by changing the carbon black content and the thickness. The real part (epsilon') and the imaginary part (epsilon") of permittivity of the blends were calculated from the S-parameters measured by a network analyzer. The reflection loss was simulated by using epsilon' and epsilon". The reflection loss. less than -10 dB could be obtained in the frequency range of 9.6similar to13.5 GHz from the sample with 10 wt% of carbon black at 1.9 mm of thickness.