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Microwave attenuation materials with high thermal conductivity are required for developing high power microwave technology. In this paper, boron-doped diamond films with different doping concentrations were prepared by microwave plasma chemical vapor deposition. Complex permittivity of the samples was measured in the K-band by using the transmission/reflection method. It was found that the complex permittivity of the diamond films increased with an increase in the boron doping concentration and the diamond films were transformed from a microwave transparent material into a microwave absorbing material. In addition, although thermal conductivity decreased with increasing boron concentration, it remained at a fairly high level. Therefore, boron-doped diamond films could be developed into a microwave attenuation material with extremely high thermal conductivity. Mechanism analysis revealed that the increase in the real part of permittivity mainly resulted from the hopping polarization of bound charges, while the increase in the imaginary part was due to both hopping polarization and valence band conduction. It was found that with the increase in the boron doping concentration, the proportion of the dielectric loss of the first mechanism increased. [ABSTRACT FROM AUTHOR]