Polyurethane (PU) matrix reinforced with carbon nanofillers forms a nanocomposite, which in addition helps to attain unique material properties by the use of the extreme high aspect ratio and nanoscopic dimensions of the nanofillers. Multi-walled carbon nanotubes (MWCNTs) are used as the nanofillers with weight percentage (wt%) ranging from 1% to 10%. The electromagnetic properties of the nanocomposites are then extracted and these show that the novel nanocomposite is a potential candidate for various microwave applications. MWCNT functionalized with Iron (Fe3O4) is used to prepare CNT/PU nanocomposites in three distinct concentrations, with lower weight percentage of 5% Fe3O4 and higher weight percentage of 20 % Fe3O4. The effect of Fe3O4 on the electrical conductivity is then analyzed and the results show the undesirable oxidization effect of Fe3O4, which significantly reduced the electrical conductivity of the composites. The oxidization effect is verified by comparing the performance of CNT-Fe3O4/PU composites with Fe3O4 powder/PU composites.