A carbon nanotube/polyaniline composite nanotube with a diameter of 50—80 nm has been prepared. The carbon nanotubes encapsulated by polyaniline (PANI) with a thickness of 20—30 nm have been synthesized by in-situ emulsion polymerization. The microwave permittivity of the carbon nanotube/polyaniline composite nanotubes has been investigated in the frequency range of 2—18 GHz. The real part (ε′) and imaginary part (ε″) of the complex permittivity of carbon nanotube/polyaniline composite nanotube change little, as compared with those of carbon nanotubes at frequencies 2—18 GHz. The ε′ and ε″ of the carbon nanotube/polyaniline composite nanotube are small at low frequencies, which make it easy to match the impedance of free-space in the design of microwave absorbing materials. The dissipation factor tanδ(ε″/ε′) of carbon nanotube/polyaniline composite nanotube is high. So the carbon nanotube/polyaniline composite nanotube is a good microwave absorbent.