A compact rectangular waveguide TE10 to TE20 mode converter is proposed in this paper. It consists of a dominant mode waveguide and an overmoded waveguide connected with coplanar and codirectional configuration to promise a compact cross section. A septum is inserted into the overmoded waveguide to divide it into two channels. The converter utilizes a Riblet-hybrid-like structure and two humps inserted in narrow walls of each of the two channels. The inherent 90-degree phase of the hybrid together with another 90-degree phase difference between the two channesl created by the humps can realize half-wavelength phase relation and excite TE20 mode. The cross section size of cavity structure of the mode converter is about 1.52 λ ×0.61 λ at center frequency of 9.5 GHz. Tested results demonstrate that within a fractional bandwidth of 3.3%, the mode conversion efficiency is better than 97.7% with a return loss of larger than 20 dB. The peak power capacity is larger than 470 kW at common air condition.