Abstract: In order to meet the demand for calibration of circular waveguide couplers for S-band high-power microwave on-line measurements, a high-efficiency S-band rectangular waveguide TE₁₀ mode to circular waveguide TM₀₁ mode converter is developed in this paper. The mode converter structure is simulated and optimized using CST software, and the performance of the developed mode converter is measured in both the frequency and time domains. The simulation and measurement results in the frequency domain show that the mode converter operates at a center frequency of 2.1 GHz, with S₁₁ < −20 dB and S₂₁ > −0.1 dB in the operating bandwidth of 60 MHz. Measurements in both frequency and time domains show that the insertion loss of the mode converter is less than 0.1dB at 2.1 GHz, indicating the high conversion efficiency of the mode converter. With the mode converter developed in this paper as the core and the commercialized waveguide coaxial converter, the circular waveguide TM₀₁ mode exciter has been successfully used in the performance calibration of circular waveguide couplers.