Activated carbons were prepared from rubber wood sawdust by chemical activation using ZnCl2 as an activation agent at 500 °C for 60 minutes with ZnCl2/dried rubber wood sawdust mass ratios from 1.0 to 2.0. Flat polyurethane (PU) composites filled with the activated carbons were prepared by a chemical foaming method using different loading amounts of the activated carbons to investigate their complex permittivity and the microwave absorption properties for use in electromagnetic interference (EMI) shielding. It was found that the best activated carbon is obtained at a ratio of 1.5, which has the highest Brunauer-Emmett-Teller surface area and a micropore volume of 1301 m2/g and 0.37 cm3/g, respectively. With increasing activated carbon content, the dielectric constant (ɛ’) and the return loss increase in the frequency range of 1-3 GHz. The composite filled with 8 wt% activated carbon has a maximum dielectric constant of 3.0 and its return loss is above 10 dB at the global system mobile phone frequency of 1.8 GHz. Its EMI shielding efficiency is in the useful range of approximately 3 dB over a wide frequency range of 1-2.5 GHz. Compared with conventional materials such as polyethelene and polyester filled with metal additives, this composite is suitable for microwave absorption and is a potential candidate for EMI shielding applications.