Abstract: Objective Aiming to address the issue that operating efficiency and service life of high-power gyrotrons are directly affected by the selection of microwave window materials, hermeticity of brazed joints and dielectric properties after sealing, three internationally recognized high-power microwave window candidates, sapphire (single-crystal Al₂O₃), AlN and diamond, are investigated in this study, so as to provide a reference for the development of microwave windows used in high-power long-pulse gyrotrons. Methods Three window materials are bonded to copper substrates using Ag-Cu-Ti active brazing technology. The vacuum leak rate, mechanical property and microstructure of the brazed joints are compared and analyzed. The dielectric constant and dielectric loss tangent of the brazed windows are accurately measured by the quasi-optical resonant or method in the extremely high frequency (30～300 GHz) bands, including W band (75～110 GHz) and D band (110～170 GHz). Results Three microwave window materials exhibit significant differences in applicable frequencies, the optimal operating frequency of sapphire windows is 95～97 GHz, that of AlN ceramics is 100～102 GHz, and diamond is more suitable for the D band and higher frequencies. Conclusion Microwave window materials should be selected according to practical index requirements, so as to give full play to material advantages and achieve the best comprehensive performance. The results of this study can provide an effective technical reference for the development of microwave windows for high-power long-pulse gyrotrons.