Abstract: To achieve the high-efficiency mode conversion of an overmoded coaxial waveguide to a two-way rectangular waveguide in high power microwave systems, an overmoded coaxial waveguide for two-way rectangular waveguide power divider is researched. The divider uses the radial line as a mode transition structure from a coaxial waveguide to a rectangular waveguide. First, the coaxial waveguide TEM mode is divided into four rectangular waveguide TE₁₀ modes; then the four rectangular waveguides are combined into two paths, which effectively inhibit the generation of high-order modes in the overmoded waveguide and achieve a high power-handling capacity and transmission efficiency. The design and experiment results show that at a central frequency of 2.88 GHz, the VSWR of the power divider is approximately 1.04, the corresponding transmission loss of the TEM mode to the TE₁₀ mode is 0.15 dB, the output imbalance is approximately 0.15 dB, and the power capacity of the design is 4.52 GW, which can satisfy the needs of high-power applications.