Electromagnetic interference (EMI) shielding materials with polyimide resin as matrix and hybrid fillers (carbonyl iron and multiwalled carbon nanotubes (MWCNTs)) as fillers were prepared, and their electromagnetic and EMI properties were investigated in the frequency range of 8.2-12.4 GHz. The microstructures of the composites show a uniform dispersion of the hybrid fillers. The effective complex permittivity and the EMI effectiveness of the composites increased with increase in the MWCNTs content. When the content of MWCNTs was 4 wt %, the effective complex permittivity and EMI effectiveness could reach 66.5 + 45.5 and 24 dB. The average values of the surface reflection (SER) and the internal absorption (SEA) increased 0.7 and 2.1 dB for 4 wt % MWCNTs composites, respectively, after the composites were heat treatment at 300 degrees C. Those results indicated that the polyimide matrix composites could be used as effective EMI shielding materials.