The effects of the particle size of sendust, which is an alloy of Al 5%, Si 10%, and Fe 85%, on the absorption characteristics of composite electromagnetic wave absorbers made of polystyrene resin and sendust were investigated in the frequency range from 1 to 40 GHz. The size of senclust particles was varied between approximately 5 and 20 mu m. A metal-backed single-layer absorber made of 20 mu m sendust particles absorbed more than 99% of electromagnetic wave power at frequencies above 20 GHz. Meanwhile, a composite made of 5 mu m particles exhibited a return loss of less than -20 dB in the frequency range of not only several GHz but also above 30 GHz. In addition, the relative complex permeability mu(r)* was shown to be controlled by adjusting the particle size of sendust, and an electromagnetic wave absorber with a flexible design was proposed.