In this paper, a novel approach based on the differential evolution algorithm is proposed for the optimal design of high-power millimeter-wave TM01-TE11 circular waveguide mode converters. The axes of the waveguide mode converters bent in one plane are represented by cubic B-splines, and the coordinates of the B-spline nodes are parameters to be optimized. Numerical optimization results show that the proposed approach can be used to design highly efficient mode converters with very short converter lengths. Furthermore, the input and output of the optimized mode converters can be in the same direction. [ABSTRACT FROM AUTHOR]