A new piezo-absorbing composite was fabricated with silicone rubber and spherical or flaky carbonyl iron particles (CIPs). The complex permittivity and permeability of the composites, under variable compressive strain, were measured using a vector network analyzer in the frequency of 2–18 GHz and the reflection loss ( RL ) calculated to test the piezo-absorbing effect. The results show that, under compressive strain, the complex permittivity decreased slightly because of the breakdown of the original conductive network, whereas the complex permeability increased, but the enhancement mechanism was different. In the case of spherical CIPs/rubber composite, this was caused by the surface effect of the absorbents, and in the case of flaky CIPs/rubber composite by the orientation of the flaky particles. As the compressive strain was applied, the RL value and the absorbing band changed slightly with thickness of 1 mm or 2 mm. As the applied strains decreased the thickness of the composites, the absorption band ( RL < −10 dB) broadened and the minimum RL decreased.