A C-band standing-wave accelerating structure was developed for a radiotherapy X-ray or an electron source, which is capable of producing 6 MeV/ average 1.6 kW or 7.5 MeV/ average 2 kW electron beams by using a 2.5 MW C-band magnetron. The bi-periodic and on-axis-coupled standing-wave structure operated at pi/2 mode is adopted for a compact accelerator size. This 48 cm long accelerating structure contains 3 different bunching cells optimized for transverse focusing as well as longitudinal bunching, 14 normal cells mainly working for acceleration, and a coupler cell on which the waveguide input coupler is attached.