The high-temperature dielectric property and evolutionary behavior of multiwalled carbon nanotube/silica nanocomposite from 30 to 600°C in the range 8.2–12.4GHz were investigated. Both increasing temperature and elevated multiwalled carbon nanotube concentration are found to enhance complex permittivity, and a mechanism for this enhancement is also proposed. The calculated attenuation suggests that multiwalled carbon nanotube/silica a good candidate for high-temperature microwave-absorbing materials, and this could help in material designs with matching conditions.