The design method of mode selective directional couplers from a transverse electric mode in a circular waveguide to the fundamental mode in a rectangular waveguide is presented in this paper. The design results for a TE01 circular mode coupler intended for use in the output power measurement of a high power gyroklystron are reported; these results indicate that more than 50-80dB directivity and sufficient suppression of unwanted modes can be obtained with a coupler less than 0.7 m in length. The effects of frequency and rectangular waveguide size on coupler parameters are also discussed in this paper.