The effectiveness of electromagnetic interference (EMI) shielding was measured for poly vinylidene fluoride (PVDF) coating materials containing carbon nanofibers. Carbon nanofibers produced from the C2H4/NC73 system exhibited higher shielding effectiveness (SE) by relatively large specific surface area and high aspect ratio than those from others. When the thickness of carbon nanofibers filled PVDF coating materials varied from 25 to 50 mum, the electrical conductivity of coating materials increased sharply from 1.34 to 1.91 S/cm. However, the electrical conductivity approached a certain value with further raise of the thickness. This phenomenon denotes that a critical thickness of coating materials exists around 50 mum. The electrical conductivity and SE of coating materials decreased dramatically as the carbon nanofiber fillers were milled. It could be concluded that the decrease of the shielding effectiveness of carbon nanofiber filled composite was due to the decrease of the filler's aspect ratio by ball milling.