Electromagnetic interference (EMI) shielding refers to the reflection and/or adsorption of electromagnetic radiation by materials, which acts as a shield against the penetration of the radiation through the shield. In this study, two directional carbon fibers reinforced composites with carbon nanotubes as fillers were studied for the high frequency electromagnetic properties and mechanical properties. Carbon nanotubes were prepared by a catalytic chemical vapor deposition method with ferrocene/xylene mixture. The electromagnetic insulating properties were measured by micro strip line method in this study. Electromagnetic interference shielding properties of composites were investigated in the frequency region of 0.3-6 GHz. The experimental results indicated that the EMI shielding effectiveness of carbon composite is sensitive to the existence of CNT.