Carbon nanotube-nanostructure Al2O3-TiO2 composite powders were prepared by mixing carbon nanotubes with nanostructure Al2O3-TiO2 ceramics in an ultrasonic bath. Electromagnetic parameters of the composite material were determined at 2—18GHz. The experimental results show that the complex permititivity and loss tangent of the dielectric increase as the weight content of carbon nanotubes increases. The minimum absorption value increases firstly and then dtcreases and the resonance frequency gradually moves to the low frequency and with increase of the weight content of carbon nanotubes and the thickness of composite coatings. Carbon nanotube 7wt%-nanostructure Al2O3-TiO2 composite coatings were deposited by micro-plasma spraying. The minimum absorption value for 1.5mm thick composite coatings was -26.3dB. The frequency range for absorption value less than -10dB of 2.0mm thick composite coatings was 3.60GHz. When the 1.0mm thick composite coating was heated to 500℃, the minimum high temperature absorption value was -12.2dB and the frequency range for absorption value less than -10dB was 2.00GHz. There is a linear relationship between experimental and theoretical thickness of composite coatings:d=0.898D+0.515, where D is the experimental thickness and d is the theoretical thickness.