The insertion loss, return loss and electromagnetic shielding effectiveness of carbon-black and carbon-fiber-filled polypropylene composites were studied in the 8 to 12 GHz frequency range. Both electromagnetic and mechanical measurements were performed at various filler concentrations. The maximum shielding values obtained for carbon-fiber-filled composites was over 40 dB for 35 wt% concentration. Young’s modulus, ultimate strength and elongation at break with respect to filler concentration were also determined. The stiffness and brittleness of the composites increased with increasing filler concentration. The ultimate strength of the carbon-black-filled composites increased whereas the ultimate strength of carbon-fiber-filled composites decreased with the filler content. Elongation at break values for all composites dropped sharply with the filler concentration.