A new centre-excited type of rectangular TE10to circular TE01mode transducer has been devised and its characteristics have been investigated both theoretically and experimentally. This transducer is an improved form of the directional-coupler type. The centre excitation—the magnetic excitation along the axis of a circular waveguide—excites few modes except the circular TE0mones (wheremis an integer), so that the coupling of the rectangular TE10mode to the circular TE01mode can be tightened and that to unwanted modes decreased. Design formulae, effects of unwanted modes and measured characteristics of the new mode transducer are given. The insertion loss of this transducer was from 0.35 to 0.7 dB over a 2 Gc/s band in the region of 24 Gc/s, which is considered to be satisfactory. This transducer is expected to have wide-band characteristics.