In order to provide wood-plastic composites (WPCs) with a GHz electromagnetic wave absorbing function, WPCs were manufactured by adding magnetic powder. The effect of increases in volume fraction of Mn-Zn magnetic powder on bending, hygroscopic, and electromagnetic characteristics of the WPCs were investigated and the electromagnetic wave absorbing performance was estimated, focusing especially on the effects of moisture on performance. The results showed that there was no deterioration in bending performance by either reducing the volume fraction of wood powder or increasing that of magnetic powder. Equilibrium moisture contents (EMC) measured at high humidity were less than 6% even for the specimen with the highest volume fraction of wood powder. Because of such a low hygroscopic property, there was no significant difference in electromagnetic wave absorbing performance, estimated using relative permittivity and permeability between the samples exposed at high humidity and in the bone-dry condition. Furthermore, return losses of double-layered WPCs for practical use were simulated by transmission line theory. As a result, matched frequencies such that return loss reaches more than 40 dB were obtained in the microwave band by optimizing the sample thickness.