Abstract: A kind of high power microwave mode converter is designed, which is lowly over-moded with input and output wave-ports perpendicular to each other. The converter can convert TM01 mode in a circular waveguide to TE10 mode in a rectangular waveguide, and works in C-band with a central frequency of 4.1 GHz. The results of simulation show that the energy conversion efficiency of the mode converter is up to 99% at 4.1 GHz, and the return loss is less than -20 dB. The bandwidth in which the conversion efficiency exceeds 90% is larger than 0.2 GHz. Its dimension is about 10 cm in each direction.