The effects of the size and shape of sendust particles on the absorption characteristics of composite electromagnetic wave absorbers made of polystyrene resin and sendust were investigated. We also investigated the difference in the absorption characteristics between permalloy and sendust. The sendust particles were granular or flakes and those of permalloy were granular. The size of sendust particles was varied in the range from approximately 5 to 20 μm. A metal-backed single layer absorber made of a composite containing small sendust particles absorbed more than 99% of electromagnetic wave power in the frequency range from 1 to 3 GHz. The values of the real part μ' r of the relative complex permeability μ* r for both magnetic materials became less than unity and had a minimum value at frequencies above 10 GHz. The composite made of small sendust particles exhibited a return loss of less than -20 dB at frequencies near 35 GHz for a suitable sample thickness.