AbstractIn this study, the electromagnetic (EM) performance of rice husk ash (RHA) calcined from rice husk was evaluated. Porous RHA with a bulk density of 0.4 g cm-3 is mainly composed of carbon and silica. The degrees of surface melting and destruction of porous structure increase with ashing temperature, while a nitrogen atmosphere retards surface melting and destruction because of the retention of carbon. A temperature of 700 °C is the lower limit for achieving significant surface melting, whereas 800 °C in air will destroy the porous structure and conductive network formed by surface melting. All RHA samples showed low permeability values caused by the absence of magnetic components. Sufficient conductive carbon and the formation of conductive networks are prerequisites for reasonable complex permittivity values. The calculated EM wave absorption of RHA achieved a maximum reflection loss (RL) of −21 dB at 2 to 18 GHz, including 5 GHz with an RL < −10 dB, which is above the average of traditional absorbers. This study reveals a new approach for fabricating an EM wave (microwave) absorber with low density and strong absorption by using agricultural wastes as starting materials.