A new method for designing a directional coupler form the TE01 mode in a circular waveguide to the TE10 mode in a rectangular wavegnide is presented. This method is used to design a - 50 dB coupler for use in the sampling measurements of a very high power gyroklystron. By varying the rectangular waveguide size the suppression of the unwanted circular modes except the TE11 mode can be increased and by modifying the distance between the last two coupling holes the coupler directivity can be maximized. By using the coupling function Φ(x) to make a second group of coupling holes we can greatly improve the suppression of the TE11 mode. Some couplers with parameters d≥100dB, p≥100dB for the TE11 mode and p>10 dB for most of the other unwanted modes were designed by this method.