Abstract: The high power microwave(HPM) sources of peak power at GW class have the characteristics of large volume and heavy weight. In the application of HPM directional radiation, the microwave sources are generally static, and with the beam waveguide to adjust the beam pointing, the transmission efficiency is usually 40% to 50%. The radar rotary joint used in the conventional mechanical scanning radar cannot meet the requirements of HPM antenna because of its small power capacity. The relations of conversion between different modes and radii of curvature in bending waveguide were investigated using the coupled wave theory. A waveguide bent 90° and a simple TM01 mode rotary joint were developed and tested. The results indicate that their transmission efficiencies were greater than 93% and the pow