Based on the mode coupling theory, the design of the geometric configuration for an TM01TE11,TE01TM11,TE0nTE01 and TE01TE11 overmoded circular waveguide mode converter with different phase rematch and structure is given to obtain more acceptable geometry parameters. The regulation of the radius of the circular waveguides, the curvature and frequency of bent circular waveguides, and the bandwidth also are obtained. With the results, a compact, efficient and wideband circular waveguide mode converter can be designed.