Conductivity, complex permittivity, and shielding effectiveness (SE) of single- and multiwalled carbon nanotube (SWCNT and MWCNT) composites prepared by ultrasonication dispersion were investigated experimentally. As compared with a manual process, ultrasonication dispersion was able to reduce percolation threshold of the SWCNT composites significantly. Frequency-dependent permittivity of the composite can be well described by the scaling law based on the percolation theory. It is also found that the carbon nanotube (CNT) composites possess large tunable dielectric properties (>60%) at relatively low bias electrical field (7.5 V/mm), which have potential applications in smart electromagnetic functional structures. SE of the MWCNT composite with 5% CNT and a thickness of 2 mm is about 30 dB at 12 GHz. Therefore, the MWCNT composites are good candidates as flexible shielding materials operating at microwave frequencies. Index Terms—Conductivity, permittivity, polymeric composite, shielding effectiveness, single- and multi-walled carbon nanotubes (SWCNT and MWCNT).