With the fast development of folded waveguide Traveling Wave Tube(TWT), there is an urgent need for attenuation materials with high thermal conductivity and good mechanical properties. In this study, boron-doped diamond film is prepared and the dielectric property of the film is analyzed. Meanwhile, the boron-doped diamond attenuator is fabricated and the thermal stability of the attenuator is investigated. The results show that the dielectric constant and dielectric loss tangent are 7.18 and 0.30 in W-band for the diamond film with boron concentration of 1.81×1019 cm -3. Moreover, the return loss and insertion loss of boron-doped diamond attenuator are 19.67 dB and 44.03 dB, respectively. With the temperature increased from room temperature to 90 ℃, the return loss and insertion loss of attenuator are increased to 20.94 dB and 45.63 dB, which demonstrates high thermal stability of the boron-doped diamond attenuator.