AbstractAn overmoded rectangular waveguide filter with the advantages of narrowband and high power is presented in this paper. The transmission efficiency of each mode in the filter is analyzed based on mode‐matching techniques and optimized by combining it with the calculation method. Due to the overmoded waveguide and direct coupling topology, the designed structure has characteristics of narrowband and high power. The overall size of the proposed filter is only 17.45 × 36.20 × 126.67 mm3 (0.54λ0 × 1.12λ0 × 3.93λ0, λ0 is the wavelength of the center frequency in free space). The test results match well with the simulated ones and they both show that the transmission efficiency of TE10 mode is higher than 99.77%. Meanwhile, the proposed filter achieves 1% fractional bandwidth and 134.5 kW power capacity, which reveals that the filter has both narrow‐band and high‐power characteristics. The designed filter would be an attractive candidate for HPM systems.